The Coating Inspectors Handbook - M

The Coating Inspectors Handbook - M
The Coating Inspectors Handbook
Tom Swan
M-TEST
2115 FM 1960 East #4
Humble, TX 77338
Phone: 281.359.2215
FAX: 281.359.2218
Cell: 281.300.9435
tswan@m-testco.com
R5.0
05/15
The Coating Inspectors Handbook
The purpose of this manual is not to turn you into a coating inspector, but to be a resource that
a trained coating inspector can use. The assumption is that you have a basic understanding of
coating inspection. Because of the global nature of coating inspection, advancements in
inspection equipment, new types of coating systems, changes in standards and new standards
organizations, it can be difficult for a coating inspector to keep current with all the changes.
This manual will try to bring together most of the current resources.
While the basics will be covered, for those that want a better understanding of the principals
behind some of the tests, there will be additional details given. It is my belief that the more
knowledge an inspector has, the better have or she can do their job. There are many good
resources out there to help the inspector to do their job better and hopefully this manual will
be one more useful tool to help you do your job better,
There is a QR code at the end of each section. Use any QR code scanner and it will take you to
the page on the website that contains the tools discussed in that section
If you have any questions or comments or would like to see an additional topic added, please
feel free to contact me.
CONTENTS
SECTION 1: Inspection
SECTION 2: Measuring Relative Humidity and Dew Point in the Field
SECTION 3: Surface Preparation of Metals – Profile
SECTION 4: Surface Preparation of Metals – Visual Cleanliness
SECTION 5: Surface Cleanliness – “Invisible Contaminants”
SECTION 6: Measuring Wet Film Thickness (WFT)
A: Mixing and Thinning
SECTION 7: Nondestructive Dry Film Thickness (DFT)
SECTION 7A: Measuring Coatings On Concrete
SECTION 8: Holiday (or as the Brits say Porosity)
SECTION 9: Tape (Peel) Adhesion:
SECTION 10: Tensile Adhesion
THE COATING INSPECTOR
SECTION 1
There are a lot of misconceptions out there on what a coating inspector is. Part of the confusion is many
people are “trained” coating inspectors but it is not the core part of their job. Paint Salesmen,
Engineers, Consultants, Managers, Contractors, all may have coatings inspection training, but in addition
to being a coating inspector, bring other perspectives to the job. Many of these individuals may get
involved with the inspections of coatings during applications, but often have a different role than
Quality Assurance for the painting.
Simply, a coating inspector’s role is to verify that the quality of work being preformed meets the
specifications and complies with any contract documents for the project. In a strict sense, judgment of
the work should be based only on the documentation and no variance should be made without
discussions with the owner or engineer that wrote the project documents despite what the inspector
may feel is OK.
Before we discuss what a coating inspector is or isn’t, we need to understand the fundamental
difference between Quality Control and Quality Assurance.
Quality Control is the roll of the person doing the work. In the paints and coatings area, this is generally
a contractor. The Contractor may employ a coating inspector to insure their coatings comply with the
specifications or if lacking specifications comply with general industry standards for good coating
practices. Often quality control is in conflict with getting the job done and completed in the time
specified or required. In the long run, it is generally better to put the priority on quality control instead
of on time and money requirements, but in the real world, even the best contractors will sacrifice some
quality for the sake of the bottom line dollar. Unfortunately, some owners will often put getting their
systems back on line over quality and may force the contractor, sometimes over the warnings of the
engineer or the inspector, to take whatever steps are necessary to complete the project on time.
If a coating Inspector is working for a contractor as their quality control, it often puts him in a difficult
position. It is the quality control person’s responsibility to take measurements of the contractors work
and record them as part of the quality control process. Contractors sometimes want the measurements
to be more in line with what they want instead of what they are.
It is important to understand that any documentation the contractor has, may in the future, be used to
determine the quality of the work should a failure occur. If the form requires a signature, and you are
the one signing the form, you may be called to give a deposition or to testify in court. It doesn’t happen
often, but don’t count on it. The moral and legal questions on putting incorrect numbers on inspection
reports is beyond the scope of this manual, but it is a matter you should consider when working for a
contractor.
Quality Assurance is typically done by someone besides the contractor. In industrial coating situations,
this is often the responsibility of a third party inspector. Quality Assurance assumes that Quality Control
has already been performed and that the contractor has released the work for inspection. It is the
coating inspector’s job to verify that the work complies with the project documents and specifications.
If the contractor does not have a quality control person, the inspector becomes both quality control and
quality assurance. This can greatly increase the load on the inspector since if the work is not correct, the
same areas may have to be inspected numerous times.
Responsibilities of a Coating Inspector
A coating inspector’s responsibility is to verify that the work conforms to a set of specifications. All work
must be documented. If it is not documented, it didn’t happen. Inspection reports should be written so
that someone that is not present during the work can recreate the project. The inspection reports can
become legal documents. All reports should be complete, legible, signed and dated.
The inspector should not require the contractor to perform work that is outside the specifications even
if it seems to make sense. The inspector should also avoid making judgments about the coatings that
are not part of the specifications. Because someone is a coatings inspector does not make them
qualified to make engineering or consulting decisions. If there are any concerns, they should be brought
to the attention of the owner, the specifer, the coating supplier or other qualified person. Making
suggestions or offering opinions if you are not qualified can have serious legal ramifications.
Sometimes the term “Coating Inspector” is used in a generic sense and may involve much more than just
inspection verification. Depending on the qualifications of the inspector and the needs of the owner,
sometimes the inspector is required to act as a project manager and have responsibilities beyond
normal inspection. He may have the authority to give the contractor permission to vary from the
specifications, stop the contractor from continuing work until problems are corrected or make other
decisions that are beyond the scope of inspection. With increased authority comes increased
responsibility and possible liabilities, including warrantee issues and other legal liabilities.
If you do not carry the proper liability insurance, be careful that you are not taking on more
responsibilities than you are prepared for. As an inspector, you owe it to the owner, the contractor and
to yourself to not get involved in projects where you may not be qualified.
As an owner, make sure that if you are hiring an inspector, you do not require more of them than they
are qualified for. Just because an inspector is NACE Certified or certified by some other training, does
not make them qualified to make decisions outside verifying the work complies with the specifications.
While many Consultants and Engineers are Certified Coating Inspectors, it should not be assumed that
consulting, failure analysis, and knowledge to grant variances from specifications are within the skill set
of a coating inspector.
Inspection Preparation Procedures
Whether you are working directly for the owner or working for an inspection company, the inspection
work should begin prior to the job. You need to determine the needs of the owner, the skill level of the
contractor, study the contract documents, find out the coating systems to be applied. Most of this you
should be able to find out prior to showing up on the job.
Once you have the job, either the company you are working for or yourself needs to get all project
documentation. The following items are useful to gather.
1. All Specifications related to the work
a. Make sure you have all addendums and appendices
2. All applicable codes and standards
3. Manufacturers Product Data Sheets (PDS) and Material Safety Data Sheets (MSDS)
4. Manufacturers Application Bulletins
5. Any drawings of area where work is being performed
6. Contact information for the Owner, Engineer, Contractor, Paint Salesman and Technical Service
representative
Project Planning
Preconstruction Meeting and Contractor Submittals
Often coatings are part of a larger project. When beginning a large project, there is generally a
preconstruction meeting where all contractors that are involved in the project are assembled to go over
the project plans and specifications. At this meeting any questions pertaining to the specifications are
discussed. Often, the coating contractor is left out of these meetings so the “more important” areas can
be discussed. If the coating contractor is present, often coatings are treated as minor work and are not
discussed in length.
A separate preconstruction meeting should be held to discuss the coatings part of the project separate
from other contractors unless the will be directly affected. If a meeting is not planned, request one.
Any possible problems with the specifications, conflicts between the specifications and product data
sheets, safety or compliance issues need to be resolved prior to the start of the project. If the
Contractor wants to change anything, this is the time. Not when the problems already exist.
Prior to the meeting, the contractor should be required to submit a written work plan or submittal. The
work plan should be specific for the project and not just a generic plan. This will spell out what the
contractors quality control plans are, detail any necessary safety or environmental issues, and detail any
areas where the contractor is free to choose among several choices.
The advantage to having the contractor put his plan is writing has two purposes. If there is a
discrepancy between his submittal and the specification or other work documents, you find out before
the project begins and if the contractor says he will do it, it is much easier to avoid arguments when you
can use the contractors own submittals.
Any changes agreed to during the preconstruction meeting should be put in writing. Copies should be
distributed to all parties. Request that they sign the copy and return it. If arguments arise later in the
project on the agreed changes, the documentation exists.
Inspection Plan
With all the project documentation as well as the contractors’ submittal, you now have all the necessary
information to compile an inspection plan. The inspection plan can be in two parts. A general project
checklist to make sure everything is completed (Figure 1) and an inspection summary sheet to
summarize all the critical parameters for the project (Figure 2).
Useful Project Checklist
PROJECT PREPARATION CHECKLIST
Job Name _____________________________
Location _______________________________
1. Documentation Checklist for an Inspection Project
1.1.
1.2.
1.3.
1.4.
1.5.
1.6.
1.7.
1.8.
1.9.
1.10.
Submittal Requested from Contractor
Submittal Signed and Returned
Preconstruction Meeting Agenda
Preconstruction meeting minutes template
Completed minutes
A copy of the specification, Product Data, and MSDS
Copy of all pertinent standards (ASTM, NACE, ISO) Inspection Task Summary Sheet
Inspector Pre-job Hazard Assessment
Job Information Sheet – Project Contact Sheet
Final report
Date _________________
Inspection Summary Sheet
INSPECTION TASK SUMMARY
Specification No.
Project Location/Name:
Item(s) to be Coated:
Title:
Rev. No.
Project Engineer:
Phone:
Coatings Inspector:
Phone:
Coatings Inspector:
Phone:
Coatings Inspector:
Phone:
Work to be performed:
Contractor:
Contractor:
Subcontractor:
Subcontractor:
Contact Person:
Client:
Phone:
Phone:
Phone:
Phone:
Phone:
Phone:
Fax or email:
Fax or email:
Fax or email:
Fax or email:
Fax or email:
Fax or email:
Utilities/Equipment/Material:
To be Provided By: Check (√ )
Surface
Preparation
Contractor
List
Owner
Power
Abrasive Blast Cleaning
Water
Blast Media & Profile
Scaffolding
Ambient Requirements
Lighting
Other Preparation Methods
Coating
Test Blast Area
Permits
Containment Equipment
Storage
Application
Other
Other
Coating System
1st Coat
2nd Coat
3rd Coat
4th Coat
Product
Manufacturer
Tech Rep
Data Sheets
MSDS
Min/Max Temp.
Ambient Requirements
Recoat Times
Thinner
Pot Life
Dry Film Thickness
Recoat Times
Cure Test
Holiday Test
Inspection Equipment/Materials Required
Test
EXAMPLE: Dry Film Thickness
Notes and Comments
SSPC PA-2
Method
Equipment
Positector 6000
5th Coat
Necessary and/or Useful reference Materials
SSPC Redbook Volume 2 or set of Surface Preparation Standards.
ASTM Volume 6.01
NACE Recommended Practices
ISO 8501:2007
SSPC Redbook Volume 1
ASTM Volume 6.02
SSPC-VIS 1 - Guide & Reference Photographs for Steel Surfaces Prepared by Dry Abrasive Blast Cleaning
SSPC-VIS 2 - Standard Method of Evaluating Degree of Rusting on Painted Steel Surfaces
SSPC-VIS 3 - Guide & Reference Photographs for Steel Surfaces Prepared by Power & Hand-Tool Cleaning
SSPC-VIS 4/NACE VIS 7 - Guide and Reference Photographs for Steel Surfaces Prepared by Waterjetting
SSPC-VIS 5/NACE VIS 9 - Guide & Ref Photos for Steel Sfcss Prepared by Wet Abrasive Blast Cleaning
Coating and Lining Inspection Manual – SSPC Publication #91-12
Clemco Blast OFF 2 – Your Guide to Safe and Efficient Blasting
Inspection Hold Points:
The number and type of Hold Points will vary from project to project, however, some hold points are
pretty much standard for most projects. These include:
Prior to the start of work
Inspection of steel
Sharp Edges
Welds and Weld Splatter
Taping off Equipment
Check Abrasives
Check Air Supply
After surface Preparation
Surface Contamination
Visual Cleanliness
Immediately prior to application of coatings
Climatic Conditions
Materials Inspection
Observation of mixing coatings
Following the application of each coat
Dry Film Thickness
Holiday Testing
Recoat Time
Coating Cure
Following the cure of the coating final inspection and sign-off
Dry Film thickness
Holiday Testing
Additional hold points can be added based on the requirements of the project.
Inspection Report
The best way to maintain credibility as a coating inspector is to act in a professional manner and to
present your documentation in a clear and concise manner. It is alright to use a standard inspection
form for most jobs but customizing the forms to match the project can increase your professionalism in
the eyes of the client and at the same time make your job easier. Diagrams can prove useful in
identifying areas of work and should be used when appropriate. Several examples are provided, but
since many situations are unique, it may be up to the inspector to design their own report. Often the
owner of or the engineer can provide drawing of the equipment being coated which can be included in
the report.
While it is not necessary to type reports, this can add professionalism and credibility to the reports. If
reports are not typed, they should be neat and legible. A report that cannot be read is the same as not
having a report.
If a deficiency is noted on a report, the correction of the deficiency should be on a subsequent report.
The correction should refer back to the report where it was noted giving the date of the report and page
and section if applicable.
With digital cameras being cheap and easy to use, when possible, photographic documentation should
be included. When working in some area, taking photographs can be difficult or impossible due to
restrictions by the owner. The inspector should use his judgment along with discussions with the owner
to determine if photographs should be part of the report.
Some inspection equipment manufacturers are making it easier to generate reports by making some of
the process automatic. You can incorporate notes and pictures and generate a report for the customer.
Report Distribution
Inspection reports should be provided to the Contractor and the owner on a daily basis. It is useful to
get the contractor to sign the report so he cannot claim at a later time he was not aware of any
problems. It is important to let the contractor know that signing the report is like signing a traffic ticket.
It does not indicate that they agree with the report, only that they have received the report. The time to
discuss signing the inspection reports during the preconstruction meeting so if there are any objections
they can be discussed. It should be made clear that this is to protect both the contractor and the
inspector.
Disagreements with the Contractor
It is inevitable at some point in a coating inspector’s career that disagreements will arise between the
Inspector and the contractor. Avoid arguing with the contractor. Some contractors are quick to try to
argue their point. Listen to what they have to say. Make sure you understand their point of view. One
way to ensure your understanding is to restate the problem as a question. “If I understand your
position, you are saying that …”. Once you understand the problem let them know you understand
their viewpoint and suggest that you work together to resolve the problem. While there is no guarantee
this approach will work, jumping right into an argument is not likely to resolve the problem.
If you had the preconstruction meeting, many of these problems should be avoided and can be resolved
using the contractors own submittal documents.
Dispute resolution
The simplest disagreement is when the contractor and the inspector get different measurements for the
same test. As a professional, you should be able to resolve who is correct.
Step one: acknowledge the problem and let the contractor know you want to work with them to
resolve the problem.
Step Two: Suggest a solution to compare the instruments they used with your equipment. Compare
both instruments with the same standard or area to resolve they are reading the same.
Step Three: Observe their Quality Control person taking measurements with their equipment and allow
their person to observe you. If the measurements are still different, further investigation needs to be
done by the inspector to determine the cause.
Step Four: Once the difference in readings has been resolved, discuss the steps you took to resolve the
problem with the contractor and the results of your investigation. Hopefully at this point you will have
resolved your problems with the contractor and demonstrated that your only goal in life is not to make
his life miserable. Best case scenario is you have demonstrated that you want to work with him to get
the job done and the worst case scenario is you avoided getting into an argument with the contractor.
If no resolution can be found for the problem, the next step is to get the owner involved. The contractor
works for the owner. Explain the problem to the owner presenting both your argument along with
appropriate documentation and why the contractor disagrees. Often the owner is not knowledgeable
about coatings and is more interested in getting the job done then in having it done according to the
specifications. The owner may side with you or he might side with the contractor. If he sides with the
contractor, thoroughly document the problem and that the owner allowed the contractor to proceed
with the work without correcting the deficiency. Since the owner approved the work to continue, a
Nonconformance Report should not be issued.
Nonconformance Reports (NCR)
Nonconformance reports can be a serious issue and should not be written unless absolutely necessary.
In some places Nonconformance reports can keep contractors from being eligible to bid on some
government projects. When a deficiency is found, as long as the contractor corrects the deficiency, no
NCR should be written up. A NCR should be issued when the contractor passes the point where the
deficiency can be corrected. i.e. Surface Profile is insufficient and the contractor has proceeded
applying the prime coat without correcting the problem.
Inspection Equipment
To properly perform your duties as a Coating Inspector requires you have properly working inspection
equipment. Equipment required for a specific project may vary but the below list should be sufficient
for most projects.
1. Electronic Relative Humidity Temperature Meter – Note: much more accurate than a sling
psychrometer.
2. Surface Temperature gauge if not included in Psychrometer – Recommend thermocouple type
or Infrared Type.
3. Dry Film Thickness Gauge –
a. Type 1 Banana Gauge – Positest FM or equivalent with calibration plates or foils
b. Type 2 Electronic Gauge –Positector 6000 or equivalent with calibration foils
4. Flashlight
5. Inspection Mirror or Boroscope
6. Magnifying Glass or digital microscope
7. Salt Testing Equipment (Conductivity or Chlorides)
8. Black light for Oil Contamination (Will not catch all oils and lubricants)
9. Blotter Paper or clean white rag for testing air supply
10. Surface Profile
a. Test-Ex Tape & Spring Micrometer or Positectot RTR Probe (Replica Tape Reader)
b. Surface Profile Gauge – Positector SPG
c. NEW Positector RTRP Surface profile – Peak Density probe
11. Holiday Tester if appropriate for the project
12. VIS Standards appropriate for the project
13. Paint Thermometer
14. Optional – Blast Needle Gauge and Nozzle Orifice gauge
15. Optional – Camera if Appropriate for the project
SECTION 2: Measuring Relative Humidity and Dew Point in the Field
Many coating failures have been attributed to applying coatings when climatic conditions
were not within specifications. When trying to determine Relative Humidity and Dew Point
temperatures, an understanding of the wet bulb, dry bulb, relative humidity and dew point is
useful in getting accurate values.
Typically, most project requirements are a Relative Humidity below 85% and a minimum 5oF
between the surface temperature and the dew point. When Relative Humidity is around 50%
and the Dew Point spread is 10oF to 15oF, accuracy in the tests are not critical. However,
when the Humidity is close to 85% (or whatever the requirement is) and the dew
point/surface temperature spread is about 5oF, it is important that readings be accurate.
There are two basic methods of measuring Relative Humidity and Dew Point Temperatures
in the field. These are addressed in ASTM E 337, Standard Method for Measuring Humidity
with a Psychrometer (The measurement of Wet and Dry Bulb Temperatures). One is with a
sling psychrometer and the second is with the newer electronic meters.
It is generally assumed that the most accurate method of determining Relative Humidity and
Dew Point are the Sling Psychrometer. Sling psychrometers used by meteorologists are
“laboratory grade” and have much greater accuracy than sling psychrometers typically used
by inspectors and contractors. Even with laboratory grade sling psychrometers, the
expected error is in the 5% to 7% range (ASTM E337-84) and it would be expected to see
even greater errors with the psychrometers typically used on coating projects. The sling
psychrometer measures two parameters, Dry Bulb (ambient temperature) and Wet Bulb.
The dry bulb temperature (DBT) or ambient temperature is the temperature of the air. This
is the temperature that you would get in the shade and not the temperature in direct sun.
The wet bulb temperature (WBT) measures the temperature that results from evaporation.
It is directly related relative humidity. When moisture evaporates, it cools the environment,
reducing the temperature slightly. The WBT will vary with Relative Humidity (RH). When
the relative humidity is high, evaporation is low and there is less of a cooling effect. When
relative humidity is low (air is dry) evaporation increases and the cooling effect is greater.
The difference between the wet bulb and dry bulb temperature therefore gives a measure of
atmospheric humidity.
Relative Humidity (RH) is the measure of how much moisture is in the air divided by the
amount of moisture the air can hold times 100. The amount of moisture the air can hold is
dependent on the atmospheric pressure. When the air is 100% saturated, evaporation will
stop and the Dry Bulb Temperature will be equal to the Wet Bulb Temperature.
When DBT – WBT = 0 then RH = 100%
It is strongly suggested that electronic meters be used instead of sling psychrometers for the
best accuracy. If you are going to use a sling psychrometer, it is recommended that the
following procedures be followed to minimize any errors..
SLING PSYCHROMETER.
- The first item is to make sure the thermometers are reading correctly.
1) Inspect the thermometers. Today, most inspectors are using the red spirit thermometers
because they do not contain mercury which is considered to be a very toxic heavy metal.
Make sure that the column is not separated. Often, especially when left in the heat or after
shipping, the red column will separate. This will result in an inaccurate reading. Sometimes
by putting the thermometer in ice water followed by hot (not boiling) water, the column can
be fixed. If it cannot be fixed, replace it.
2) Calibrate the thermometers – sometimes, thermometers do not read exactly how they are
marked.
a. Always field check thermometers. With the wick removed and in the shade, both
thermometers should read the same.
b. By definition, ice water (not ice) is always 32oF. Put ice cubes in a glass of water. Allow
time for the ice and water to reach equilibrium. 15 minutes should be safe. Place
thermometers in the water and they should read 32oF. Since the scale is linear, if it is off you
can add or subtract the difference to get the thermometer to read accurately. Example: If the
thermometer reads 33oF in the water it is reading 1oF high. This needs to be subtracted from
each reading to get the correct value. If the temperature between thermometers is off by
more than 2 oF, replace the thermometer.
- Store the Thermometers properly between uses:
If the psychrometer is stored in the sun or heat, the plastic case will become heated. When
you go to take your readings, the temperature being radiated by the sling psychrometer
case will affect your reading introducing an error. Keep the sling stored in the shade near
the temperature you will be testing in. NOTE: Thermometers stored in a trunk or car can get
hot enough to pop the top off the thermometers.
- Check the Wick:
The wick should be white, not yellow, brown or black. The wet bulb thermometer measures
the rate of evaporation of water between the surface of the wick and the bulb of the
thermometer. The wick should be clean and flexible. Often, when onsite, you use whatever
water is available. This water often has dissolved solids and impurities that get left behind as
the water evaporates. Eventually the wick becomes non-porous and while the wick may feel
wet, evaporation is effected giving erroneous readings for wet bulb temperature.
When possible use distilled water and when the wick becomes discolored or hard, cut it off
and put a fresh part of the wick on the bulb. (Caution, if you continuously use hard water in
the reservoir, the unused part of the wick can also become discolored and hard. It is always
recommend to use distilled or demineralized water to maximize the life of the wick)
- Check the Water
When using a new wick, make sure it is soaked thoroughly. If you put a drop of water on the
wick, it should not bead up. Let the wick sit for a few minutes to make sure it is saturated.
- How to Check Readings
It is important to take readings in the same area that is to be painted. Face the wind if there
is any and rotate the sling psychrometer at about 2 revolutions per second for about 90
seconds. Read the wet bulb temperature first. Be careful to keep your fingers off both
thermometers. Continue to rotate for 20 to 30 seconds and take a second reading. If the wet
bulb has not changed you are finished. If it is decreasing, continue until you get two
readings the same.
Accuracy of the method
Read the Dry Bulb Temperature before you start “Slinging” the psychrometer. Read the dry
bulb at the end of the procedure. They should be the same. If they are not, the accuracy of
the readings should be questions. The accuracy of the method depends on the accuracy of
the thermometers as well as the operators’ procedures. Let’s assume your reading is off by
one degree in opposite directions on the wet bulb and the dry bulb.
Example:
Measured WBT = 58oF DBT = 72oF therefore Wet Bulb Depression (WBD) = 14oF
RH = 42% DB = 47oF
Actual WBT = 57oF DBT = 73oF therefore Wet Bulb Depression (WBD) = 16oF
RH = 35% DB = 44oF
A 2oF error makes a 16% difference in Relative Humidity and a 6% difference in Dew Point.
Electronic Meters:
Electronic meters come is several varieties from meters that just provide Wet Bulb and Dry
Bulb Temperatures to meters such as the TQC Dewcheck, that measure, Wet Bulb, Dry
Bulb, Relative Humidity, Dew Point, Surface Temperature, Calculate the ∆T between the
surface temperature and the dew point., electronic time and date stamp data and can
download information to a computer.
As for all electronics, the quality of the sensors is key to how well the meter works. Some of
the early meters as well as some still manufactured today, suffer from using low cost
sensors that can give erroneous readings and have given electronic meters a bad
reputation. Be careful that the great price you got on an electronic meter doesn’t affect the
accuracy of the meter.
Electronic meters have some distinct advantages over sling psychrometers. Because there
are no moving parts, you can take readings close to where you will be doing the work.
Atmospheric conditions at or near the surface of steel can be considerably different inches
or feet from the surfaces. Because a sling psychrometer requires room to “sling” it, you can
never get readings near the surface.
Electronic meters also minimize operator error. When multiple inspectors or quality control
personal use an electronic meter, they should all get the same readings.
Some meters have calibration kits you can use to verify accuracy. Check before purchasing
that this can be done in the field. In most cases, even the cheapest electronic meters will be
more accurate than using a sling psychrometer.
As with the rest of industry, keeping electronic records of projects will not only become the
norm, it will be required by many owners. Make sure the electronic meter has the capability
of time and date stamping data as well as sending the data to a computer.
The typical costs for a sling psychrometer are $80.00 to $100.00 based on the model.
Replacement thermometers are generally $30.00 to $40.00 and replacement wicks are
about $1.00 to $2.00 each. While may sling psychrometers come with a slide rule calculator
on their side, for better accuracy, it is generally recommended to use a psychometric table
which you can get for under $10.00 or you can get the information for free at:
http://www.srh.noaa.gov/epz/?n=wxcalc (NOTE: While this is a government website, like all
web sites the address is subject to change- working as of June 2015)
The cost for electronic meters varies from $99.00 to $800.00. The higher priced models
generally have higher quality sensors, measure surface temperature, record readings and
will interface to a printer or computer.
Good quality electronic meters and sling psychrometers can both supply accurate
information when used properly. It is important to make sure that your readings are
accurate, especially if they are going to restrain the contractor from painting. It is probably a
good idea to keep both on site in case your batteries die in your meter or you break a
thermometer and don’t have a replacement.
Spreadsheet Formula to Calculate Relative Humidity and Dew Point.
Since most of us have computers, if you prefer to set up a spreadsheet to calculate Relative
Humidity and Wet Bulb rather than using tables, the following spreadsheet will calculate
them.
A
1
2
3
4
5
6
7
8
9
10
B
Dry Bulb (T)
Wet Bulb (Tw)
Convert T (F) to T( C)
Convert Tw (F) to Tw(C)
es
ew
e (Vapor Pressure)*
RH (%)
Td (C)
Convert Td (C) toTd(F)
C
Enter Dry Bulb Temperature Here (F)
Enter Wet Bulb Temperature Here (F)
=5/9*(C1-32)
=5/9*(C2-32)
=6.112*EXP((17.67*C3)/(C3+243.5))
=6.112*EXP((17.67*C4)/(C4+243.5))
=C6-(1015*(C3-C4)*0.00066*(1+(0.00115*C4)))
=C7/C5*100
=(237.7*(LOG(C5*C8/611))/(7.5-(LOG(C5*C8/611))))
=9/5*C9+32
NOTE: Station Pressure of 1015 mb (approx 30 inches of Hg)
SECTION 3: Surface Preparation of Metals – Profile
Surface preparation can be broken down into two main categories.
•
•
Surface Profile
Surface Cleanliness
Surface profile is the determination of the roughness of the surface and for painting purposes
involves depth of the profile, peak density and angularity of the profile.
Surface Cleanliness involves determining how much of the original mill scale, rust and paint
have been removed from the surface as well as how much invisible surface contamination is
present usually in the form of salts. More on Surface cleanliness can be found in section 4.
Surface preparation is one of the main cause of most paint failures because more than any
other factor, it affects how well the paint sticks to the surface being painted.
Most paint forms a mechanical bond with the steel and generally surfaces that have roughness
will supply the best mechanical bonds. Also, when you put a profile on the surface, you increase
the surface area, so the paint has more surface area to adhere to. Different paints are made for
different texture surfaces from smooth to rough.
Also, remember, paint will bond to the surface being coated and if the surface is loose (rust, mill
scale or old paint), when the surface breaks off, so will the paint. Some paints are formulated to
coat over these surfaces with minimal surface preparation, but they should be used with caution
and understanding
On Metal, is there a difference in profile due to sand, shot or grit?
Sand
Grit
Shot
The above drawings are rough approximations of the type of profile you might get from Sand,
Grit or Shot. The profile can vary due to many different factors, however, generally sand has a
finer profile than grit and shot gives a rounded, “pinged” type of profile. The above drawings all
have about the same profile depth, but have an entirely different appearance.
Because of the difficulty in measuring peak density, it is rarely measured for industrial coating
applications, however, the peak density can be an important factor in determining the bond of
the coating to the substrate. In general, sand will have the “highest” peak density and shot the
“lowest”. When a specification calls for an “angular” surface profile, this is generally best done
with Grit.
DeFelsko has developed a new instrument that makes measuring peak density easier and
quicker, so it is worth taking a second look as this often ignored parameter. Below are four
“pictures” of surface profile taken with the new RTRP probe. Basic parameters are displayed
directly on the meter and other parameters can be obtained by using free or paid for third party
software.
Blasted with G50
Blasted with S230/G40
Blasted with Garnet
Blasted with Bristle Blaster.
Z-axis enhanced for clarity –
The figure below is a simplified example of why BOTH peak height AND peak
density are important to the understanding of coating performance. The two surfaces
have different geometries yet their height measurements are the same. To get a clearer
picture of the surface available for bonding, peak count measurements must also be
obtained. Furthermore, both measured values make it possible to investigate the
increase in surface area resulting from the abrasive blasting process.
1.5 peaks per mm
3 peaks per mm
Figure 4: Both surfaces have the same measured peak-to-valley height.
A second important measurable parameter, peak density, helps explain why coatings
bond differently to each surface.
Pictures from “CORRELATION OF REPLICA TAPE MEASUREMENTS TO ESTABLISHED MEASUREMENT
TECHNIQUES”, Dave Beamish, DeFelsko Corporation – Link to full article:
http://www.defelsko.com/technotes/profile/surface-profile-and-adhesion.htm
Surface Profile Depth
An “exact” profile depth cannot be determined over the surface of the substrate because the
depth of the Peaks and Valleys varies greatly. Surface profile attempts to find the AVERAGE or
MAXIMUM depth for the peaks and valleys over a given area based on the method used..
Measuring Surface Profile
There are several accepted ways to find surface profile and each one has advantages and
disadvantages. The First Three methods are detailed in ASTM D4417, “Standard Test Method
for Field Measurement of Surface Profile of Blast Cleaned Steel”
1.
2.
3.
4.
5.
Surface Profile Visual Comparator (D4417 Method A)
Surface Profile Gage – Profilometer (D4417 Method B)
Press-o-film Testex® Tape (D4417 Method C)
Defelsko RTRP Surface profile – Peak Density Gauge
Surface Roughness Tester
Surface Profile Visual Comparator: - Method A
There are several different visual comparators. The most common one in the US is the KeanTator comparators which come in sand, shot and grit. For ISO, according to ISO 8503 part 1,
Surface Comparator you can use the TQC LD2040 and 2050, There is also the Rugotest (TQC
LD6010) no. 3 comparison standard for blasted surfaces consisting of 6 examples of grit-blasting
and 6 examples of shot-blasting.
These comparators are cast in metal to approximate surface profiles up to 4 mils. The
comparators are held up to the blast profile and usually viewed with a lighted magnifier (5X or
10X) and the texture and roughness of the comparator is compared to the blasted steel.
Because the profile changes based on the media used, comparators are sold as Sand, Shot, or
Grit.
Comparators, when used by an experienced inspector should give accuracy to approximately
±0.5 mils within the range of the comparator. Since not all grit gives the same profile, this is
probably the most problematic comparator to use. Until the operator is experienced, it is usually
best to “calibrate” their eyes using one of the other methods to confirm readings. Since the
readings are subjective and there is no record with this method, I do not recommend it for field
use where different abrasives may be used.
Comparators are better suited for shop use where the same abrasives and conditions are
repeated. Care should be exercised when new personnel are using them and frequent checks
should be made by an experienced inspector or by another method until they are properly
trained.
A Surface Profile Gage – Method B
These were the original surface profile gauges. This is a gage with a wide base that sits on the
peeks and has a needle that goes into the valleys. Since these gauges measures only a single
point, only one peak to valley reading is made with each reading. To get a good idea of the
average surface profile, several measurements must be made and averaged together. Method
B requires a minimum of 10 readings per spot averaged together. The meter must be zeroed to
a smooth surface, such as glass. With a dial gage, readings must be recorded as they are
made. There is no permanent record.
With the emergence of electronics, the original surface profile gauges have been made easier
by automating the process of averaging the readings. These gauges such as the Defelsko
Positector SPG are digital and will record average, min and max readings and download to a
computer for a Permanent record. It also has wireless (Bluetooth) built in to the it. To date, this
is the most accurate and reliable method to determine the “accurate” surface profile. It also
allows you to keep a full record of measurements. (While it does not conform to the current
ASTM method, 5 readings can be taken instead of 10 with little loss in statistical integrity)
The Positector 6000 SPG also has a TestEx mode that simulates the same reading as you
would get using TestEx tape by dropping the low readings and only recording the max reading.
You can access this feature in the Advanced model using SmartBatching.
Press-o-film Testex® Tape – Method C
Testex Tape is probably the most common method used to determine surface profile. The Tape
has a compressible foam layer with a 2 mil Mylar covering. A “Burnishing” tool (Most people call
this a swizzle stick), is used to rub the foam into the profile. The foam takes on the shape of the
profile and it is measured with a spring micrometer. Since the foam is covered with 2 mils of
Mylar, this must be subtracted from the reading to get the surface profile.
It is important to understand that TestEx tape measures maximum profile and not average
profile. When using Testex Tape, make sure the area is clean and representative of the area
being tested. If the tape is not rubbed with sufficient pressure, the correct reading will not be
achieved. As the center of the tape is rubbed, the color will change slightly. The entire surface
should look the same.
If the tape is reading within 20% of the maximum or minimum reading on the Testex tape, the
next higher or lower tape should be used.
The advantages to this method are:
Gives the profile over approximately 3/8 inch area.
1. Gives a permanent record of the test.
2. Easy to do.
3. No objectivity on the part of the operator.
Disadvantage.
1. Can get costly if many measurements are required
2. Improper “Burnishing” of the tape can give low results.
3. Since the micrometer reads the thickest area, it will give a number closer to the
maximum value rather than the average.
4. Use of the wrong grade of tape can give the wrong answer.
5. Repeated readings can compress the tape giving wrong readings.
Testex comes in several grades.
Course E122-B - Testex Tape - 50 Tape Roll Coarse - 0.8 - 2 mils
Extra Course E122-C - Testex Tape - 50 Tape Roll Extra Coarse - 1.5 - 4.5 mils
Extra Course+ E122-F - Testex Tape X-Course Plus - 1.5 to 8 mils
NOTE: There is a new optical grade for use with the Defelsko RTRP
Spring Micrometer verses Defelsko Replica Test Reader
As previously mentioned, TestEx tape is not accurate in the upper 20% and lower 20% of its
range. This is do the way the Spring Micrometer measures the tape. The New Defelsko RTR
probe linerises the Testex tape measurements so it is accurate through the full range of the tape
grade Below graph was developed by DeFelsko.
What if I am on the job and I don’t have a “Burnishing” Tool?
For those of us without a dictionary, Webster’s Dictionary defines burnishing as; “to rub (a
material) with a tool for compacting or smoothing”
To acquire the most common burnishing tool, the easiest thing is to break for lunch and go to a
restaurant that serves mixed drinks. Request a Swizzle Stick and you have a burnishing tool. It
should have a round end and be deburred prior to use. The ASTM method does NOT define
what a burnishing tool is. The most common object used is generally the end of a disposable
pen. The Press-O-Film directions say to use the edge of the container that holds the tape.
What is used as a burnishing tool is not as important as to make sure the surface is “burnished”
thoroughly. Not rubbing hard enough can lead to erroneous profile readings.
We have a question on a job from several years ago and the tape was included as part of
the job record. Will it still read the same?
The answer is; it should. Once the foam is compressed, it should hold the original profile
indefinitely. It is possible that frequent readings or if heavy objects were placed on it, it could
compress further and give low readings.
Surface Roughness Tester
This is a relatively new method and because of its complexities, generally is not used as a field
test. The method involves moving a diamond stylus over the surface to be measured. The meter
records max and minimum peaks and valleys, number of peaks, peak density on several other
variables. The meter can be interfaced to a computer or give a direct printout. These meters
start around $1,200 to $2,500 and are generally used in shops that have quality control
departments.
The downside is that the diamond stylus may be too large to make into the valleys of the profile
and may not give an accurate profile. It will however, give an accurate peak density count.
If the blast profile measures too high or too low. What can we do?
If the profile was measured with a comparator, you could be getting a bad reading due to using
the wrong comparator or even if using the correct comparator, the differences between the blast
patterns on the metal may be to different to get an accurate reading. Use Method B or C to
confirm.
If a profile gage was used, make sure it is zeroed properly and if you have a surface with a
known profile, test it for accuracy. If it is still off after taking several readings, make sure the tip
is still in good shape. While the tip is very hard, it is also brittle and dragging the tip or dropping
the meter on the tip, may cause it to break.
If you used Testex tape, make sure the surface was clean before you use the tape. Dirt on the
back of the tape will give erroneous readings. Make sure the tape was rubbed hard enough and
with a proper burnishing tool. You thumbnail may not provide sufficient pressure. Make sure the
proper grade of TestEx tape was used. You can also use Method B to confirm readings.
If two different methods are used and you cannot get them to agree, testing has found the
Surface Profile Gauge, when used properly, gives the best results.
If the blast profile is confirmed and it is still out of spec. Some general trouble shooting ideas
may include the following:
1. Check Air Pressure using a needle pressure gage at the blast nozzle.
2. Check Blast Nozzle Orifice size using an orifice gage.
3. Are you using the proper type and size of media.
4. Are you over recycling the media.
5. Run a screen test to determine the actual size of the media
6. Is the blast operator running the equipment properly.
7. Adjust the pressure at the blast nozzle is possible.
8. Change how far the operating is standing off the nozzle.
9. Increase or decrease the angle of the blast on the steel.
10. Are you blasting a previously blasted profile that is different than what you are trying to
achieve.
A complete discussing of Abrasive Blasting is beyond the scope of this question and a
consultant should be contacted if you are having problems.
How does Surface Profile Affect Paint Usage?
The rule for paint is 1 gallon of paint applied at 1 mil will cover 1,604 sq ft. wet film thickness on
a smooth surface. The effect of blast profile is important in calculating estimates of paint
quantities required especially in cases in which the specification requires application of a
minimum dry film thickness. Given a series of peak to valley heights of an abrasive blast
cleaned surface, the greater the peak to valley height, the more paint will be required to fill the
profile before a measurable thickness of paint is applied.*
Figure #1
Figure #2
In the above drawings, if Figure #1 represents a 1,604 sq ft area and we needed a 2 mil coating
of 100% solids paint, we would need to apply 2 gallons of paint.
It should be fairly easy to determine that Figure #2 has a greater surface area than Figure #1. If
Figure #2 is the same area with a Sand or Grit blast profile, You would APPROXIMATE 3
gallons of paint to get 2 mil Dry Film Thickness (DFT). See Rule of Thumb Below.
Rule of Thumb:
A rule of thumb for the determination of the approximate extra paint required to fill a SAND or
GRIT blast profile is to multiply the peak to valley height (profile) times 0.5 and add this to the
Dry Film Thickness you are trying to achieve..
For example, for a peak to valley height of 10 mils, an additional quantity of paint equal to a full
coat at 5 mils dry film thickness will be required. Because a SHOT Blast Profile is smoother, the
amount of paint would be slightly less and you might want to use 0.25 times the blast profile to
calculate the additional amount of paint.
*NOTE: See next question for explanation of “before a measurable thickness of paint is
applied.”
Understanding the Effect of Profile Effect on DFT Measurements?
If you calculate the Wet Film Thickness required to get a dry film thickness, if you don’t allow for
the profile, you will be off. When you measure a Wet Film Thickness you are measuring the
amount of paint ABOVE the peaks.
Zero Point for Wet
Film Thickness
When using a DFT Gage the meter must establish a zero point. When you have a surface with
peaks and valleys, there is no clear line where the “Zero Point” is. There are areas in the profile
that are LESS THAN THE ZERO POINT. The meter will not register any paint as being applied
to the surface until it is greater than the zero point.
Zero Point for Dry
Film Thickness
In the above drawing, if the DFT gage perceives the “Zero Point” to be above the orange paint,
any paint below the dashed line will not be measured. This will be explained in greater detail in
the section on Dry Film Thickness.
Applicable Surface Profile Standards
Standards are available from several organizations that provide direction for using various
methods to obtain an anchor profile measurement. Available standards include but are not
limited to the following:
ASTM D 4417: Standard Test Methods for Field Measurement of Surface Profile of Blast
Cleaned Steel
ASTM D 7127: Standard Test Method for Measurement of Surface Roughness of Abrasive Blast
Cleaned Metal Surfaces Using a Portable Stylus Instrument
NACE RP0287: Standard Recommended Practice – Field Measurement of Surface Profile of
Abrasive Blast Cleaned Steel Surfaces Using a Replica Tape
SSPC PA-17: Procedure for Determining
Roughness/Peak Count Requirements
Conformance
to
Steel
Profile/Surface
ISO 8503-1:2012: Preparation of steel substrates before application of paints and related
products – Surface roughness characteristics of blast-cleaned steel substrates – Part 1:
Specifications and definitions for ISO surface profile comparators for the assessment of
abrasive blast-cleaned surfaces
ISO 8503-2: Preparation of steel substrates before application of paints and related products –
Surface roughness characteristics of blast-cleaned steel substrates – Part 2: Method for the
grading of surface profile of abrasive blast-cleaned steel – Comparator procedure
ISO 8503-3: Preparation of steel substrates before application of paints and related products –
Surface roughness characteristics of blast-cleaned steel substrates – Part 3: Method for the
calibration of ISO surface profile comparators and for the determination of surface profile –
Focusing microscope procedure
ISO 8503-4z: Preparation of steel substrates before application of paints and related products –
Surface roughness characteristics of blast-cleaned steel substrates – Part 4: Method for the
calibration of ISO surface profile comparators and for the determination of surface profile –
Stylus instrument procedure
ISO 8503-5: Preparation of steel substrates before application of paints and related products -Surface roughness characteristics of blast-cleaned steel substrates – Part 5: Replica tape
method for the determination of the surface profile
SECTION 4: Surface Preparation of Metals – Visual Cleanliness –
Surface preparation is probably the main cause of most paint failures because more than any
other factor, it affects how well the paint sticks to the surface being painted. Surface preparation
can be broken down into two main categories.
•
•
Surface Profile
Surface Cleanliness
Surface profile is the determination of the roughness of the surface and for painting purposes
involves depth of the profile and angularity of the profile. . More on Surface cleanliness can be
found in the Surface Preparation – Profile – SECTION 3
Surface Cleanliness involves determining how much of the original mill scale, rust and paint
have been removed from the surface as well as how much invisible surface contamination is
present usually in the form of salts. More on Surface cleanliness can be found in the Surface
Preparation – Salts – SECTION 5
Visual Cleanliness
Pint will bond to the surface being coated and if the surface is loose (rust, mill scale or old
paint), when the surface breaks off, so will the paint. Some paints are formulated to coat over
these surfaces with minimal surface preparation, but they should be used with caution and
understanding.
Because there is not specific test for Visual Cleanliness, standards have been developed to
determine specific levels of cleanliness. The most common visual standards are SSPC/NACE
and ISO. Since the determination is visual, guides have been established to help clarify the text
in the specifications. A brief summary follows.
SSPC-SP 1 – Solvent Cleaning
This specification covers the requirements for the solvent cleaning of steel surfaces. Removal of
all detrimental foreign matter such as oil, grease, dirt, soil, salts, drawing and cutting
compounds, and other contaminants from steel surfaces by the use of solvents, emulsions,
cleaning compounds, steam or other similar materials and methods which involve a solvent or
cleaning action.
SSPC-SP 2 - Hand Tool Cleaning
This specification covers the requirements for the hand tool cleaning of steel surfaces. Removal
of all rust scale, mill scale, loose rust and loose paint to the degree specified by hand wire
brushing, hand sanding, hand scraping, hand chipping or other hand impact tools or by a
combination of these methods. The substrate should have a faint metallic sheen and also be
free of oil, grease, dust, soil, salts and other contaminants.
SSPC-SP 3- Power Tool Cleaning
Specifies the use of power assisted hand tools to obtain a steel surface free of all loose mill
scale, loose rust, loose paint, and other loose detrimental foreign matter. It is not intended that
adherent mill scale and rust be removed by this process. Mill scale and rust are considered
adherent if they cannot be removed by lifting with a dull putty knife
.SSPC-SP 5/NACE No. 1 - White Metal Blast Cleaning
This standard covers the requirements for white metal blast cleaning of steel surfaces by the
use of abrasives. Removal of all mill scale, rust, rust scale, paint or foreign matter by the use of
abrasives propelled through nozzles or by centrifugal wheels. A White Metal Blast Cleaned
Surface Finish is defined as a surface with a gray-white, uniform metallic color, slightly
roughened to form a suitable anchor pattern for coatings. The surface, when viewed without
magnification, shall be free of all oil, grease, dirt, visible mill scale, rust, corrosion products,
oxides, paint, or any other foreign matter.
SSPC-SP 6/NACE No. 3 ISO 8501 1-1: 1988(E) (SIS 05 59 00) Sa 2 - Commercial Blast
Cleaning
This standard covers the requirements for commercial blast cleaning of steel surfaces by the
use of abrasives. Removal of mill scale, rust, rust scale, paint or foreign matter by the use of
abrasives propelled through nozzles or by centrifugal wheels, to the degree specified. A
commercial blast cleaned surface finish is defined as one from which all oil, grease, dirt, rust
scale and foreign matter have been completely removed from the surface and all rust, mill scale
and old paint have been completely removed except for slight shadows, streaks, or
discolorations caused by rust stain, mill scale oxides or slight, tight residues of paint or coating
that may remain; if the surface is pitted, slight residues of rust or paint may by found in the
bottom of pits; at least two-thirds of each square inch of surface area shall be free of all visible
residues and the remainder shall be limited to the light discoloration, slight staining or tight
residues mentioned above.
SSPC-SP 7/NACE No. 4 -Brush-Off Blast Cleaning
This standard covers the requirements for brush-off blast cleaning of steel surfaces by the use
of abrasives. Removal of loose mill scale, loose rust, and loose paint, to the degree hereafter
specified, by the impact of abrasives propelled through nozzles or by centrifugal wheels. It is not
intended that the surface shall be free of all mill scale, rust, and paint. The remaining mill scale,
rust, and paint should be tight and the surface should be sufficiently abraded to provide good
adhesion and bonding of paint. A brush-off blast cleaned surface finish is defined as one from
which all oil, grease, dirt, rust scale, loose mill scale, loose rust and loose paint or coatings are
removed completely but tight mill scale and tightly adhered rust, paint and coatings are
permitted to remain provided that all mill scale and rust have been exposed to the abrasive blast
pattern sufficiently to expose numerous flecks of the underlying metal fairly uniformly distributed
over the entire surface.
SSPC-SP 10/NACE No. 2 - Near-White Blast Cleaning ISO 85011-1:1988 (E) (SIS 05 59 00) Sa 2 1/2
This standard covers the requirements for Near-White Metal Blast Cleaning of steel surfaces by
the use of abrasives. Removal of nearly all mill scale, rust, rust scale, paint, or foreign matter by
the use of abrasives propelled through nozzles or by centrifugal wheels, to the degree hereafter
specified. A Near-White Blast Cleaned Surface Finish is defined as one from which all oil,
grease, dirt, mill scale, rust, corrosion products, oxides, paint or other foreign matter have been
completely removed from the surface except for very light shadows, very slight streaks or slight
discolorations caused by rust stain, mill scale oxides, or light, tight residues of paint or coating
that may remain. At least 95 percent of each square inch of surface area shall be free of all
visible residues, and the remainder shall be limited to the light discoloration mentioned above.
SSPC-SP 11 - Power Tool Cleaning to Bare Metal –
Specifies the use of power tools to produce a bare metal surface and to retain or produce a
surface profile. This specification is suitable where a roughened, clean, bare metal surface is
required, but where abrasive blasting is not feasible or permissible. Once cleaned, the surface
will be free of visible oil, grease, dirt, dust, mill scale, rust, paint, oxide, corrosion products, and
other foreign matter. Slight residue of rust and paint may be left in the lower portion of pits if the
original surface is pitted. Surface shall have a degree of roughness (profile) of no less than 1 mil
(25 microns). Although the MBX Bristle Blaster exceeds the minimum surface profile, at the
current time, it falls under this standard.
SSPC-SP 12/NACE No. 5: Surface Preparation and Cleaning of Steel and Other Hard
Materials by High-and Ultrahigh-Pressure Water Jetting Prior to Recoating
This standard provides requirements for the use of high- and ultrahigh pressure water jetting to
achieve various degrees of surface cleanliness. This standard is limited in scope to the use of
water only without the addition of solid particles in the stream.
SSPC-SP 13/NACE No. 6 - Surface Preparation of Concrete
This standard gives requirements for surface preparation of concrete by mechanical, chemical,
or thermal methods prior to the application of bonded protective coating or lining systems. The
requirements of this standard are applicable to all types of cementitious surfaces including castin-place concrete floors and walls, precast slabs, masonry walls and shotcrete surfaces.
An acceptable prepared concrete surface should be free of contaminants, laitance, loosely
adhering concrete, and dust, and should provide a dry, sound, uniform substrate suitable for the
application of protective coating or lining systems. Depending upon the desired finish and
system, a block filler may be required.
SSPC-SP 14/NACE No. 8 – Industrial Blast Cleaning
This joint standard covers the use of blast cleaning abrasives to achieve a defined degree of
cleaning of steel surfaces prior to the application of a protective coating or lining system.
Industrial blast cleaning provides a greater degree of cleaning than brush-off blast cleaning
(NACE No. 4/SSPC-SP 7), but less than commercial blast cleaning (NACE No. 3/SSPC-SP 6).
Industrial blast cleaning is used when the objective is to remove most of the coating, mill scale,
and rust, but when the extra effort required to remove every trace of these is determined to be
unwarranted.
The difference between an industrial blast and a brush-off blast is that the objective of a brushoff blast is to allow as much of an existing coating to remain as possible, while the purpose of
the industrial blast is to remove most of the coating.
A commercial blast is free of mill scale, rust, and coatings, and allows only random staining on
less than 33% of the surface. The industrial blast allows defined mill scale, coating, and rust to
remain on less than 10% of the surface and allows defined stains to remain on all surfaces.
SSPC-SP 15 Commercial Grade Power Tool Cleaning
This standard covers the requirements for power tool cleaning to provide a commercial grade
power tool cleaned steel surface, and to retain or produce a minimum 25 micrometer (1.0 mil)
surface profile. A commercial grade power tool cleaned steel surface, when viewed without
magnification, shall be free of all visible oil, grease, dirt, rust, coating, oxides, mill scale,
corrosion products, and other foreign matter, except as noted.
Random staining shall be limited to no more than 33 percent of each unit area of surface as
defined. Staining may consist of light shadows, slight streaks, or minor discolorations caused by
stains of rust, stains of mill scale, or stains of previously applied coating. Slight residues of rust
and paint may also be left in the bottoms of pits if the original surface is pitted.
This standard differs from SSPC-SP 3, Power Tool Cleaning, in that a higher degree of surface
cleanliness is required, and a minimum surface profile of 25 micrometers (1.0 mil) will be
retained or produced. This standard differs from SSPC-SP 11, Power Tool Cleaning to Bare
Metal, in that stains of rust, paint, or mill scale may remain on the surface.
SSPC-SP 16, Brush-off Blast Cleaning of Non-Ferrous Metals
SP 16 is intended for brush-off blast cleaning of coated or uncoated metal surfaces other than
carbon steel prior to the application of a protective coating system. Surface preparation using
this standard is intended to roughen and clean coated and uncoated non-ferrous metal
substrates, including, but not limited to, galvanized surfaces, stainless steel, copper, aluminum,
and brass. SP 16 requires the cleaned surface to be free of loose contaminants and loose
coating as determined by visual inspection. A minimum surface profile of 19 micrometers (0.75
mil) on the bare metal surface is required. Intact coatings are required to be roughened to the
degree specified in the project specification.
SSPC-PA 17, Procedure for Determining
Roughness/Peak Count Requirement
Conformance
to
Steel
Profile/Surface
SP-17 is intended for use by specifiers and contractors. It provides a method for determining
whether the profile of a steel surface is in conformance with project specifications when using
the instruments and procedures contained in ASTM D 4417 and D 7127. Requirements for
frequency and location of instrument readings and evaluation criteria to ensure that the profile
over the entire prepared surface complies with project requirements are included.
SSPC VISUAL STANDARDS
It is important to understand that the Guides only describes the pictorial standard and does not
constitute the standard. It is to be used for comparative purposes and is not intended to have a
direct relationship to a decision regarding painting requirements.
SSPC-VIS 1
SSPC-VIS 2
SSPC-VIS 3
SSPC-VIS 4/NACE VIS 7
SSPC-VIS 5/NACE VIS 9
Guide and Reference Photographs for Steel Surfaces Prepared by Dry Abrasive
Blast Cleaning
Standard Method of Evaluating Degree of Rusting on Painted Steel Surfaces
Guide and Reference Photographs for Steel Surfaces Prepared by Power- and
Hand-Tool Cleaning
Guide and Reference Photographs for Steel Surfaces Prepared by Waterjetting
Guide and Reference Photographs for Steel Surfaces Prepared by Wet Abrasive
Blast Cleaning
SSPC/NACE/ISO/Swedish Standards Chart
Steel
Structures
SSPC (USA)
NACE
British Std. Swedish Standard SIS
BS 4232
05 5900 - 1967 / ISO
8501-1 : 1988
Shipbuilding Research
Association of Japan SPSS.
White Metal
SSPC - SP 5
NACE # 1
1st Quality
SA 3
JA Sh 3 or
JA Sd 3
Near White Metal
SSPC - SP 10
NACE # 2
2nd Quality
SA 2½
JA Sh 2 or
JA Sd 2
Commercial Blast SSPC - SP 6
NACE # 3
3rdQuality
SA 2
JA Sh 1 or
JA Sd 1
Brush Off Blast
NACE # 4
-
SA 1
-
SSPC - SP 7
NOTE: In the above table, the comparisons are only approximate between Swedish/ISO standards and
SSPC/NACE standards. There are significant differences and they should not be interchanged without
understanding the differences.
I have a Specification that call for SSPC SP-X/NACE Y, what is my first step.
If you have a specification that requires abrasive blast , Hand tool, Power Tool or Water
Jetting, make sure you have a copy of the standard specified. Unless otherwise stated
in the specification, always use the most current version. If there is a VIS Guide
applicable to the method being used, it is useful to have these on site. These can be
purchased from the specifying organizations or from most Equipment distributors
including myself.
I have a Specification that requires SSPS SP6/NACE 3, do I need to do SSPC
SP1?
YES. All the surface preparation methods require removal of oil. Grease and other
contaminants by SSPC-SP1 or other agreed upon method. If you do not clean the
surface first, you do not meet the specification criteria.
The contractor says all the mill scale has been removed, but I think I still see
some?
Mill scale usually fractures and breaks off during blasting and does not accept a profile.
If there is any question, apply a couple of drops of 5% Copper Sulfate to the area in
question, Steel will turn copper colored and mill scale will not.
I am arguing with the contractor over “staining” verses “rust”?
Rust is iron oxide that is attached to the surface of the steel and must be removed. If
you have ever tried to get rust out of a shirt, you know that it can stain and does not
come out easily. This can also happen to steel. A stain is part of the metal and can only
be removed by removing the surface of the steel. Think of it like wearing a white shirt
where the shirt is the metal. You spill spaghetti sauce on your shirt, it is like rust.
Remove the sauce by rubbing with a wet cloth and you still have a red stain. It is OK to
have the sauce stain just not the sauce.
As a last resort, you can examine the surface under magnification and determine if the
stain is part of the surface or above the surface. A 10X magnifier or the 30X Pocket
microscope works great for this as does the EXTECH MC108or MC200 which also take
photographs. (Note – the method states “viewed without magnification” so use caution
when using magnification for what you are looking at)
I have a project that requires SSPC SP2 or SP3. Before I start, I can not remove
any rust, mill scale or paint with a Dull Putty Knife. Am I done?
NO. The dull putty knife test applies after the hand tool or power tool cleaning Once the
entire surface is clean, then check the surface with the dull putty knife.
Using Vis 1, the contractor says he has met the spec, I say he has not.
Remember, Vis 1 (and the other Vis guides) is a guide to help explain the standard. In
case of disputes, the text of the method determines if the surface had met the
specification. This being said, there are many factors that affect the appearance of the
final blasted surface. Make sure you are using the proper starting grade for the metal in
the Vis Guide.
Also, in the back if the Vis 1 Guide, it shows metal blasted to an SP5 with several
different media. These all meet the criteria of an SP5 but they all have a different
appearance.
My specification calls to use VIS 1 to determine surface profile. How do I do that?
You can’t. Surface profile has nothing to do with surface cleanliness. Prior to doing any
work, this needs to be addressed and you may want to refer them to a Consulting
Service since they should not be writing specifications.
Do I really need a “white metal” blast or is “near white” OK?
In general, you need a white metal blast for immersion surfaces, metalizing, and
inorganic zinc. For most other services, near white is usually OK. The coating
manufacturer should have the final determination of the required cleanliness. Also, on
new steel, there should not be any difference between a white metal and a near white
blast because there should not be any staining.
I need to achieve an SP 11 – What is the best way?
There are many considerations to determine the best method, but in general the
Montipower MBX Bristle Blaster easily achieves the 1 mil profile and generally 2.5 to
3.5 mils. It also gives the benefit of giving a profile that closely resembles that of a grit
blasted surface.
SECTION 5: Surface Cleanliness – “Invisible Contaminants” Tips and Tricks:
While many factors can lead to coating failure, perhaps the most common reason is Inadequate
Surface Preparation. It is important to understand that surface preparation has two components:
Visible
Invisible (Surface Contamination)
What are Visible and Invisible Surface Contaminants?
There is general agreement in the coating industry on the importance of visible surface
preparation and that will be dealt with in the Surface Cleanliness – “Visible” Cleanliness.
Invisible surface contamination is much more problematic, less understood and less agreed on
by many “experts”. Remember, MOST articles on this subject have been written by people with
a specific agenda in mind. I am not aware of any “definitive” article on surface contamination
written form a pure “research” perspective. Make sure you separate the HYPE from the FACTS
For additional information I will refer you to the article “Myths About Salts, chlorides, and
Coatings” - This article appeared in the April Materials Performance Magazine. Much of what
you know about salts may be wrong. You can get it on my website at www.m-testco.com.
Invisible surface contaminates are generally defined as any substance on the surface (or near
the surface) of the substrate that cannot be viewed with the eyes. These are normally Salts.
Salts primarily cause two problems:
CORROSION.
OSMOTIC BLISTERING
What are Salts?
In chemistry, salt is a general term used for ionic compounds composed of positively charged
cations and negatively charged anions, that combine so that the product is neutral and without a
net charge. These ions can be inorganic (Cl-) as well as organic (CH3-COO-) and monoatomic
(F-) as well as polyatomic ions (SO42-).
The important part is salt is composed of ions which have a negative or positive charge. When
salts are DISSOLVED in water, they form a SOLUTION. The Cations and Anions are necessary
for a current to flow in a liquid such as water. The dissolved salts are referred to as TOTAL
DISSOLVED SOLIDS (TDS). The higher the SPECIFIC CONDUCTANCE of the ions, the better
a current will flow and the easier it is to create a CORROSION CELL. In addition, the total
number of ions present in the solution, (TDS) determine the OSMOTIC PRESSURE that will be
exerted on a SEMIPERMIABLE MEMBRANE.
In order to understand the importance of Salts, it is important to understand some terms. These
definitions may make a chemistry teacher cringe, but they are intended to explain the words in
the context of this discussion.
DISSOLVED: The most common salt is sodium chloride (NA+Cl-). When salt is put into water
and stirred, it disappears. This happens because the salt breaks apart into ions, the Cation
(NA+) and the Anion (Cl-).
SOLUTION: The dissolved salt forms a solution with the water. The ions move into the water
matrix and become physically inseparable from the water.
TOTAL DISSOLVED SOLIDS: The total number of ions in a solution usually expressed in ppm.
SPECIFIC CONDUCTANCE: Different ions are more conductive than other ions. That is why
sulfuric acid makes a better battery than salt water. The higher the specific conductance of the
ions in the solution, the more charge the water can carry and the more efficient corrosion cell it
will generate. The conductivity of a solution is often used to estimate the Total Dissolved Solids.
CORROSION CELL: A picture is worth a 1000 words.
For corrosion Cell to occur requires 4 items.
1.
2.
3.
4.
Anode – Corrosion (metal loss) occurs at the anode.
Cathode – For this discussion it is sufficient to know you need a cathode.
Electrolyte – Water that contains IONS (dissolved salts)
Metallic Pathway – Substrate.
SO, for corrosion to occur requires Salts for the electrolyte. When a surface is painted, it
physically separates the electrolyte from the metallic pathway thereby preventing corrosion cells
from forming.
OSMOTIC PRESSURE (OSMOTIC CELL): Again this is best illustrated with an illustration:
Nature wants everything to be in balance and will do whatever is necessary to restore that
balance. There are Twice as many X’s or Ions on the left side of the membrane as the right
side.
To balance things out you can move 15 X’s from the left side to the right side. Unfortunately, the
X’s or ions will not go through the membrane so it must balance it out moving the water which
will go through the membrane. To balance things out, 1,000 W’s must go through the membrane
for every 60 X’s to balance it out
Since there is not enough room for the water, it pushes up on the paint and causes a blister.
Osmosis is a property of the solution and not a property of the salt. The driving force or Osmotic
Pressure is determined by the concentration of salts in the solution on the surface (substrate)
and not the concentration of salts on the liquid side.
For those that want a more technically correct definition read the following. Osmosis is a
relatively complex and the process is dependent on the concentration of the solute. For the salt
concentrations in which we are dealing Osmosis acts like a vapor phase reaction.
While most coatings provide “barrier protection” meaning the slow the movement of water to the
substrate, in reality they become saturated with water and my contain 1% to 3% moisture in the
matrix. When the water molecules reach the surface of the substrate, if the surface is free of
ions, the pure water will continue its journey and go back into the coating. However, if there are
soluble ions on the surface, they will go into solution with the water. Since the water now has
soluble ions present, the vapor pressure of the new solution is increased and the migration of
the water is slowed down. Since the amount of water flowing in is constant and the amount of
water flowing out is slowed, eventually the buildup of water will forced the coating off the surface
forming a blister. This process will continue as long as the concentration of ions below the
coating is equal to the concentration of the solution above the coating at which time the water
flowing in will equal the water flowing out.
NOTE: Osmosis is actually far more complex than the above description and if you want a
more complete description go to a good chemistry book. I would caution against going online
because the explanations are also oversimplified (and sometimes wrong) for making easier to
understand rather than being scientifically correct.
NOTE: Solvent Entrapment as well as other soluble organics can also cause blistering.
SEMIPERMEABLE MEMBRANE: Will allow molecules to pass through in the gas phase but will
not allow liquids to pass through. In coating applications, Paint can act as a semi-permeable
membrane. How permeable the paint is depends on various properties of the paint including
type, hardness, porosity, and thickness.
How much Salt is too Much?
My Rule of Thumb: The less salts the better. Some salts on the surface of the substrate to be
coated are generally not critical on surfaces that will have atmospheric exposure unless water
may pool on the surface or the surface is exposed to consistent condensation. (Remember:
Condensation and precipitation are close to pure water) The definition of “some” can be
debated. For surfaces that will be in immersion environments, it is important that the
“conductivity” of the contaminants on the surface to be coated, be an order of magnitude or less
than the “conductivity” of the liquid on the surface of the coating.
Some Specs require testing for chlorides and some for salts, are they the same?
Yes and No. The primary salt present on most surfaces is sodium chloride. Na+Cl-. The chloride
ion is one half of the sodium chloride molecule. When running a “chloride” test, the test looks
specifically for the chloride ion. Much of the time, this is sufficient to determine invisible
contamination on the surface, but it does not give total salts.
The only practical way to measure total salts is by use of conductivity. This is a backdoor
approach to measuring salts, because it measures the results of the salts and is not specific to
any particular salt. As salts dissolve in a liquid, they break down into their ionic states. The ions
increase the conductivity of the liquid so the greater the measure of conductivity, the greater the
salt level. This can be used to estimate the Total Dissolved Solids (TDS)
A conductivity sensor measures how much electricity is being conducted through a centimeter
of water. Specific conductivity is expressed as mhos per centimeter (M/cm), sometimes called
siemens per centimeter (S/cm). Because a mho (or siemen) is a very large unit, the micromho
(microsiemen) or millimho (millisiemen) typically is used (mS/cm).
The conversion factor depends on the chemical composition of the TDS and can very between
0.54 – 0.96. A value of 0.67 is commonly used as an approximation if the actual factor is not
known
TDS (ppm) = Conductivity (mS/cm) x 0.67
For values in the range sensed by most TDS meters, a rough conversion is that 1 ppm NaCl =
2.2 mS/cm.
Which is more important, Salts or Chlorides?
For most applications, total salts is more important than looking specifically for chlorides.
Chlorides have gotten a bad reputation because it is believed they are more aggressive to steel
than many other ions. For chlorides (or any other ion) to be aggressive, however, there must be
a corrosion cell present and if the surface is painted properly, a corrosion cell does not exist.
The main purpose for removing salts is to prevent osmotic blistering. Osmotic pressure depends
on the number of ions in solution and is independent of the type of ions present. Again, Osmosis
will only occur in IMERSION environments and when the CONCENTRATION of IONS on the
surface of the steel is Greater than the Concentration of Ions in the liquid on the exterior of the
coating.
It is important to note that surfaces that are in atmospheric service that are subject to pooling
rain or persistent condensation can have some of the same problems experienced in immersion
services. Rain water and condensation are relatively pure which can cause osmotic cells and
blistering if surfaces under the coatings are contaminated. IN GENERAL, when properly
painted, “some” salt or chloride contamination on a substrate is NOT A PROBLEM for
atmospheric service.
What Units are used to measure salts?
In the US the most common unit of measure for Surface Concentration is Micrograms per
square centimeter (µg/cm2)
In Europe the most common measure is Milligrams per square meter (mg/m2)
This is mass per unit area conversion is: (1 µg/cm2 = 10mg/m2)
The concentration of the salts in solution may be referred to as 1 µg/mL = 1 µg/g (of water) = 1
PPM.
Where do Salt Limit Numbers Come From in Specifications?
Every coating is different. The coating manufacturer should be able to tell you how much
surface contamination the coating can withstand. Unfortunately, very few manufacturers have a
clue so they resort to playing it safe. Most specs, that have salt or chloride limits use one of the
following limits.
No Measurable Chlorides (Salts) (SSPC SC-1)
Less than 3 µg / sq cm (Mil Spec)
Less than 4 µg / sq cm – Elcometer Recommendation in 130 Manual
Less than 5 µg / sq cm
Less than 7 µg / sq cm (SSPC SC-2)
Less than 20 µg / sq cm
Less than 50 µg / sq cm (SSPC SC-3)
REMEMBER: Since most test methods only extract, at best, 50% of the non-visible
contamination, the amount of surface contamination is actually ay least twice as high as the
amount measured. i.e. If you measure 10 µg / sq cm you probably have at least 20 µg / sq cm
on the surface being measured.
When dealing with salts on metal to be painted, less is always better. Generally, if specifications
require less than 20 µg / sq cm for non-immersion surfaces, the specifier is being cautious
unless extenuating circumstances are present. That doesn’t mean he is wrong in setting the salt
limit so low, just that he is being cautious.
When dealing with immersion surfaces, it is important that the ionic strength of the liquid on the
outside of the coating be used to determine the amount of salts that can be tolerated on the
substrate. When dealing with Demineralized or pure water, less than measurable salts should
be present on the substrate. When dealing with potable water, most commonly 7 µg/sq cm is
used.
Also the thicker, and less permeable the coating, the more salts can be tolerated on the
substrate.
My specs say salt or chloride levels shall be less than X ppm. What does this mean?
It means the person who wrote the specs doesn’t know what they are doing. Contact the owner
or spec writer prior top starting the project to determine the proper limit. To get from ppm to
µg/sq cm you need to know:
1. The amount of surface area used to collect the sample.
2. The amount of water used to collect the samples.
Equation:
What is the best way to test for chlorides or salts?
There are various methods of testing for salts. The first thing that needs to be determined is do
you need to test for total salt or chlorides. If the test method uses conductivity, you are testing
for Salts, not chlorides. If the test uses chemistry, (Quantabs, drops or Kitagawa tubes), you are
testing for chlorides.
Salt tests include the:
Bressel Test with the Conductivity Meter
Surface Contamination Test (SCAT) with a Conductivity Meter
Potassimum Ferricynide Test
Parks Salt Meter
Elcometer Salt Meter
Chloride Tests include
Chlor*Test
Bressel Test with Quantabs or Kitagawa Tubes
Scat Test with Quantabs. or Kitagawa Tubes
NOTE: The Chlor-Sleeves can be used with a conductivity meter but do NOT use the
solution that comes with the test. The solution has conductivity and will not give an
accurate reading for salts. If you use the solution you need to subtract out the
conductivity of the solution from your reading
There is also a CSN (Chloride/Sulfate/Nitrate) Test. While this is billed as a “Total
Salts Test” it is not since there are many other salts. It does test for the most common
salt Anions – Chlorides and Sulfates. Unless you are in an agricultural area – nitrates
are generally not a problem. It does not tell you what the cation attached to the anion
is.
What is the lest expensive way to run salt or chloride tests?
The least expensive test is the Scat Test. It involves drawing a 6 in x 6 in (10 cm x 10 cm)
square and swabbing it with DI water and a cotton ball. This is also the least accurate.
Elcometer Salt meter: Initial cost $4000 to $5000 plus $1.30 per test
Bresle Test: Initial Cost (by conductivity) $390.00 plus $6.00 per test
Chlor*Test: Initial Cost: $0.00 plus $17.90 per test
Potassium Ferricynide Test - $.25 to $0.50 per test
How long do the tests take?
The SCAT Test, Chlor*Test and Bresel Test take about 10 minutes per test. The complete CSN
test takes 15 to 20 minutes per test.
Potassium ferricynide take about 30 seconds per test. It measures F++ (Free ferric ions) which
will not exist without an anion. This is a yes/no test and it semi-quantitative. More blue on the
paper indicates higher levels of salt but it is relative and quantative
The Parks salt meter takes about 5 to 10 minutes per test
Which test is most Accurate?
Accuracy comes into play in the effectiveness of removing the salts to be measured from the
surface as well as the method of measurement. It is generally accepted that the SCAT Test
removes about 25% of the salts from the surface and at best the other methods remove 40% to
60% of the salts from the surface so the number you get is probably half of the amount of actual
salts on the surface being tested. Independent testing of the Elcometer SCM shows random
extraction amounts. I have not seen any test data from the manufacturer. The amount of water
in the paper at the time of measurement can affect the test results and there is not a good way
to control it, especially in hot, dry environments.
When they are run properly, Potassium Fericynide test papers are sufficient if just looking for
the presence of salts,. It needs to be run within minutes after the blasting of the steel. Since it
is actually measuring the amount of FE++, it is not appropriate for other substrates. MTEST is
one of the few companies selling the papers.
The Bresle test is currently the only test that has its own standard, ISO 8503-6 and ISO 8503-9.
When running a Bresle test, the accuracy ranges from 40% to 80% extraction based on how
long you massage the patch. When running a Bresle test, it is generally advisable to run a blank
and subtract any “built in contamination” from your results. If you follow the extraction directions
in ISO 8502-6, you should be able to achieve 80% to 90% extraction. Based on watching
operators run this test, the general extraction rate is probably about 40%.
It has been reported that some Bresle patches may contain high conductivity. These patches
are generally imitations made in Asia and if you are not sure of the manufacture, a blank should
be run to test for conductivity contributed from the patch.
While I have not seen it, I have also been told of positive tests for nitrites using the CSN kit due
to contamination of the test sleeve so if you encounter a problem with nitrates – run a blank
before proceeding.
What about determining if oil is present on the surface of the steel:
ASTM A-380 Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel
Parts, Equipment, and Systems contains specific details on use of black light for surface
inspection: 7.3.2 "Black Light Inspection is a test suitable for the detection of certain oil films
and other transparent films that are not detectable under white light. In an area that is blacked
out to white light, inspect all visible accessible surfaces with the aid of a new, flood-type,
ultraviolet lamp." However, ASTM A-380 also states that "The test will not detect straight-chain
hydrocarbons such as mineral oils." Use of Black Light for inspection of surface preparation is a
useful tool and can be a required step for certain industries.
When using this method, it works best in low ambient lighting so do it at night or turn out the
lights. Black light and US light are sometimes used the same but they are different. I find that
UV lighting at 365 nm works the best. Black light is in the 400 – 450 nm range.
Some people like putting a drop of water on the surface. When Oil is present the water tends to
ball up verses spreading out on clean steel. This test can be subjective and is only reliable for
highly contaminated surfaces.
SECTION 6: Nondestructive Dry Film Thickness (WFT)
Many contractors wait until after the paint dries before determining its thickness. While it is not
generally the inspector’s responsibility to monitor the Wet Film Thickness, it is generally a good
idea to check it when possible. This should be the responsibility of the contractor as part of their
quality control program but is sometimes overlooked. If you determine the wet film thickness is
high or low prior to coating a large area, adjustments can be made to correct the application. It
is much easier to correct the film thickness prior to letting it cure. It is relatively simple to
calculate the wet film thickness that should be applied to get the proper dry film thickness.
Theoretical Coverage
1 mil of 100% Solids Coating covers 1604 ft2/WFT
Practical Coverage
Theoretical Coverage x % Loss
Dry Film Thickness = DFT in mils
Wet Film Thickness = WFT in mils
All % are by volume and expressed as a decimal i.e. 60% = .60
DFT = WFT x (% Solids)
 % Solids 
DFT = WFT 

 (1 + %Thinner ) 
WFT =
DFT
%Solids
WFT =
DFT
WFT
%S
DFT
WFT
%S
7
8
90%
5
8
60%
6.5
6
8
8
80%
70%
4
3
8
8
50%
40%
DFT
 %Solids 
 100% +%Thinner 


Using A Wet Film Gage
1. Dip WFG into wet paint
2. Paint Wet Film Thickness is between 4 to 6 mils in
the above example.
The comb gauge pictured is the most common one in use, They may be made of stainless steel
and if properly maintained should last for many years. Less expensive ones are made of
aluminum and are good for a few uses and some are made of plastic and are used once and
thrown away.
There are other types which are more commonly used in other industries. The wet film wheel
consists of three circles. The central circle is of smaller diameter and is eccentric of the two
outer circles. By rolling the gauge through a wet coating, the centre disc eventually touches the
film. This point on the scale indicates the thickness.
Various measurement ranges are available from 0 to 25µm to 0 to 3000µm (0 to 1mil - 0 to
40mils) are available
•
•
•
Continuous Scale results in ±5% measurement accuracy
Suitable for flat and curved surfaces
Stainless steel giving a hard-wearing instrument which can be cleaned with solvents for
reuse
The Pfund Thickness Gauge consists of two concentric cylinders,
one sliding inside the other. A spherical glass lens is fitted to the end
of the central cylinder and when pressed into the wet film it leaves a
trace. The diameter of this mark varies depending on the thickness
of the coating, which can easily be assessed from the conversion
table supplied with the instrument.
•
•
Ideal for measuring the thickness of translucent products (varnish, oils etc.)
Measurement range of 2.25 - 360µm (0.09 - 14.17mils)
SECTION 6A: Mixing and Thinning
After surface preparation, mixing and thinning is probably the area that causes the most failures.
Improper thinning, the wrong thinner, the wrong mixture, incorrect induction time or exceeding
the pot life can all cause paint failures or a shorter life than the coating was designed for.
To prevent errors in mixing coatings, it is important to read and understand the product data
sheet (PDS) for the coatings being used. If you do not have access to the PDS, then contact
the coating manufacturer. Do not assume you know the proper mixing procedure and thinner.
Prior to doing any mixing, the batch number should be recorded and the date should be
checked to make sure it is in date. While it does not happen often, sometimes paint failures are
the result manufacturing errors.
If part of the coating fails and the failed coating is a different batch number than the good
coating, this is an indication it could be a manufacturing problem rather than an application
problem. Manufacturers retain batches and can recheck the coating to determine if the failure
was due to a problem with the coating.
SOLVENT EVAPORATION RATE (in minutes)
Solvent
Denatured Alcohol
Evaporation
Rate
3
VM & P Naphtha
4
Lacquer Thinner
2
Paint Thinner or
Mineral Spirits
60
Toluene
3.5
Xylene
12
Acetone
1
M.E.K.
2
Turpentine
40 5
Kerosene
325
Uses in Paint Industry
Not Used For Paints
can be used for cleaning up certain residues like masking tape or
stickered labels
It is also used in some epoxy and automotive finishes. Can be used as a
clean-up solvent for "oil-based" products and is a good brush cleaner.
a general purpose solvent used in the manufacture of most oil-based
trade sales paints. It is excellent for thinning oil-based paints
Thinner for polyesters, industrial paints and finishes. Cleaner and
degreaser. A fast evaporating solvent and thinner.
Similar to Toluene, is also strong and fast acting but evaporates at a
much slower rate than Toluene. Many oil/alkyd resins are made with
Xylene. it is too fast for most brush applications. Consequently, its use
is really limited to paints applied by spray gun and as a clean-up solvent.
Ketones are often used in maintenance paints like vinyls, phenolics,
acrylates and chlorinated rubber coatings. Acetone is a strong, fast
acting solvent, cleaner and remover for inks, resins, adhesives and
contact cement. It can be used as a clean-up after fiberglass projects.
It is a strong, fast acting solvent, cleaner and remover for inks, resins,
adhesives and contact cement.
It has a narrow range of solvency and possesses a strong odor. Its use
in coatings is very limited.
Has extremely low solvency and slow evaporation are desired. Possible
uses might include paste wood fillers and putties
The thinner that is used can greatly affect the cure of the coating. Different solvents evaporate
at different rates and the thinners used for a particular coating system are chosen so that they
evaporate out faster than the cure. If the coating cures prior to the solvents evaporating, solvent
entrapment can occur which can lead to coating delamination, blistering or improper curing. If
they evaporate to fast, the coating may skin over and wrinkles or a haze may form.
SECTION 7: Nondestructive Dry Film Thickness (DFT)
Which Dry Film Thickness Gage is Best for Me?
There are two types of nondestructive DFT generally referred to as Type 1 or “Banana Gages”
or Type 2. Electronic gages. Advantages and disadvantages of both types are listed below.
Type 1
Advantages
No Batteries
Relatively Durable
No Electronics
Type 2
Advantages
Easy to Calibrate
Can Calibrate to Base Metal
Faster Readings
Menu Driven
±1% to ±3% Accuracy
Statistics and Memory Capabilities
Downloadable to a computer
Will work with Ferrous and Nonferrous
Metallic Substrates
Disadvantages
Not as easy to calibrate
5% Accuracy
Generally not zeroed to
base metal
Easier affected by
operator procedures
Less accurate than±
electronic gages
Cannot store readings or
do statistics
More difficult to read
Works only on Magnetic
Substrates
Disadvantages
Requires Batteries
Not a Durable as Type 1 Gages
NOTE: Type 1 Gages also include “Pencil Pull-off gages” which are good for quick field
checks but generally not used for Quality Control Purposes. They are generally rated at ±10% to
±15% accuracy when used properly.
Are There any Guidelines for Using These Meters?
The most commonly used Guideline is SSPC-PA2, “Measurement of Dry Film Coating
Thickness with Magnetic Gages”.
IMPORTANT: There are significant differences between the Original PA2 (1982/91) and
the Newer Version (1997, 2004). Make sure you know which version you are working
from.
Other Standards that may be applicable are the following ASTM Methods:
ASTM B 499, “Measurement of Coating Thicknesses by the Magnetic Method: Nonmagnetic
Coatings on Magnetic Basis Metals.”
ASTM D 1186,”Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings
Applied to a Ferrous Base.”
ASTM D 1400, “Nondestructive Measurement of DFT of Nonconductive Coatings Applied to a
Nonferrous Metal Base.”
ASTM E 376, “Measuring Coating Thickness by Magnetic-Field or Eddy-Current
(Electromagnetic) Test Methods.”
ASTM G 12, “Nondestructive Measurement of Film Thickness of Pipeline Coatings on Steel.”
European Specifications generally refer to EN ISO 19840, “Paints and varnishes – Corrosion
protection of steel structures by protective paint systems – Measurement of, and acceptance
criteria for, the thickness of dry films on rough surfaces”.
Understanding PA-2
Some of the following information reprinted from Elcometer’s elconews e-zine.
•
•
•
•
For Structures not exceeding 300 sq ft, take 5 spot readings per 100 sq ft.
For Structures not exceeding 1000 sq ft, Select 3 random 100 sq ft areas to test.
For Structures exceeding 1000 sq ft, Select 3 random 100 sq ft areas to test in
the first 1000 sq ft and for each additional 1000 sq ft test one random 100 sq ft
area.
If any area is not in compliance, the non compliant area should be determined
How do I determine the minimum number of tests required?
To determine the number of tests required if the surface area is greater than 1,000 sq ft, use the
following formula to determine the minimum number of areas to test:
3 + [(SFC AREA) -1000) / 1,000] = Number of Test Areas
Number of Test Areas X 5 = Number of Spot Readings
Number of Spot Readings X 3 = Number of Gage readings
Example: 27,500 square feet to be coated to 12 – 15 mils of paint.
Round to 28,000
3 + [(28,000 – 1,000) / 1,000] = 30 areas to test
3 areas in the first 1000 sq ft and one area in the remaining 27 – 1000 foot areas
30 X 5 = 150 Spot Readings
150 X 3 = 450 Gage Readings
All Spot readings must be ±20& of range.
For a spot reading use roughly a 1 inch diameter circle. Within this, the probe is placed 3 times
in random positions (Gage Reading). The average of these 3 gage readings is a called the ‘spot
reading’
Example:
The previous example calls for a DFT range of 12 to 15 mils, the area meets SSCP PA-2 if:
The Average of all the Spot readings must fall within the 12 -15 mil range
and
• All spot reading are greater than 80% of the Specified DFT (0.8 X 12 = 9.6 mils)
and
• All spot readings are less than 120% of the Specified DFT (1.2 X 15 = 18 mils)
Therefore the PA2 Range = 9.6 to 18.0 mils for individual spot readings.
Individual Gage readings do not have a range.
Another way to look at PA2 is the thickness of the coating is acceptable if the DFT of an
area fits within the curved part of the graph.
In the current PA2, it requires mapping out of areas that are out of compliance and only
requires correcting those areas. The old method requires mapping out the entire project.
Also, the square footage can be in any shape, not just squares.
Standardization bodies, such as ASTM, BS, CEN and DIN, hold the copyright to all their
publications. Everyone wanting a Standard must therefore pay for it via the standard
bodies websites,
What if ISO is specified?
EN ISO 19840, “Paints and varnishes – Corrosion protection of steel structures by
protective paint systems – Measurement of, and acceptance criteria for, the thickness of
dry films on rough surfaces”. This standard method has special calibration techniques,
(See Procedure).
Readings of the thickness of the paint film should be taken according to a sampling
plan. The number of readings depends on the size of the inspection area and they are
taken at random points (see table).
SAMPLING PLAN
Most of the readings should be above the Nominal Dry Film Thickness (N) so the
average ( x ) will be equal to or greater than N.
• Dry Film Thickness (N) so the average ( x ) will be equal to or greater than N.
• No more than 20% of the readings in an area can be thinner than the Nominal (N) but
not thinner than 80% of nominal (0.8N).
• All readings should be less than the maximum thickness (Max).
The thickness of the coating is acceptable when readings are within the curved part of
the graph. (See Procedure). There is a provision for repeating a few readings; consult
the Standard for details.
ALTERNATIVE PLAN
Annex B of EN ISO 19840 is a variation of the above procedure, subject to agreement
by interested parties. Say 5 thickness readings are taken in a 30mm circle and the
average is calculated. The location of these little groups is according to the Sampling
Plan above. The thickness of the coating is classed as acceptable when the averages
are analyzed and they fit within the graph
I need an electronic Type 2 meter, what should I get?
For most people the Defelsko Positest 6000 is the best choice. It is designed primarily
for field usage, is light, compact and easy to use. It can be ordered with a built in
“integral” probe or with a separate detached probe. If you are only going to measure
large pieces and you are sure the model you order will meet your needs, a fixed probe
is a good choice. If you later need a seperate probe, no problem, change it out.
If you get involved with many different types of projects, need to get into small areas get
involved with coating thickness from thin to thick films, the separate meter provides the
flexibility to change with the job by just changing out the probe.
The meter comes with a ferrous(F), nonferrous(NF) or a duel FN probe. The same
meter will accept all of the probes, so if your needs change, you only need to order a
new probe.
The last decision is memory or no memory.
Standard 250 Readings in One Batch
Advanced 10,000 Readings in 1,000 Batches
For basic applications, the Standard Model wil store 250 readings and download them
to your computer or the positector.net website. For large projects, where batching is
needed, the Advanced model can saves hours of work and provide you with
professional, computer generated reports.
Both the Standard and Memory models have Bluetooth or you can dowload via an optional
cable.
How often should I calibrate the meter?
Type 1 meters should be verified each time they are used. The BMR should be
subtracted from each gage reading to get the proper coating thickness. PA2 specifies
that the Type 1 meter should be verified using “Calibration Plates” not shims or foils.
This may be changed in the new version and in reality calibrating with foils is just as
accurate.
Type 2 meters should be verified each time they are used, according to PA2 before and
after each shift. In situations where many reading are being taken throughout the day, I
recommend frequent verification of the calibration. If you take 500 readings over the
course of the day and the calibration verification at the end of the day is off, since there
is no way of knowing when it went wrong, you have to backtrack all the reading until you
find the point. It is much simpler to verify the accuracy of the meter throughout the day.
Type two meters can be calibrated with foils (shims) or calibration plates. When using
foils, calibrate with the shim on the prepared substrate. DFT is the actual reading taken.
If the meter is calibrated to plates, then the actual DFT is the Reading – the magnetic
base reading (BMR).
When using a type 2 meter on nonferrous metals, it is essential to calibrate the meter
to the base metal being tested. Calibrating the ferrous part of an FNF meter does NOT
calibrate the nonferrous probe.
What is a Base Metal Reading (BMR)?
This is probably one of the most misunderstood concepts in Coating Thickness Testing.
There are two separate properties that effect the BMR.
1. The magnetic properties of the steel
2. The surface profile of the steel
Generally, the BMR is 30% of the surface profile measurement
All meters are zeroed to steel with a certain magnetic property. Since many factors can
affect the magnetic properties of steel during its manufacture, each metal will have its
own “zero point”.
When you have a surface with peaks and valleys, there is no clear line where the “Zero
Point” is. There are areas in the profile that are LESS THAN THE ZERO POINT (Blue
Paint). The meter will not register any paint as being applied to the surface until it is
greater than the zero point.
Zero Point for Wet
Film Thickness
Zero Point for Dry
Film Thickness
In the above drawing, if the DFT gage perceives the “Zero Point” to be the top of the
blue paint, any paint in this area will not be measured.
When you calibrate a DFT Gage to a flat plate, the BMR simply adjusts the Dry Film
Thickness to the Zero Point.
Let’s look at the effect of BMR on a Type 1 & Type 2 meter with a 2 mil surface profile.
• with a Type 2 gage properly calibrated to the blasted substrate, if you measured every
possible point from peak to valley, the average DFT reading would 2.0 mils because the
meter was “zeroed” to the BMR.
• With a Type 1 gage properly calibrated to a calibration plate, if you measured every
possible point from peak to valley, the average DFT would be greater than 2.0 mils. If
the BMR was 0.75, the average measurement would be 2.75 mils, Since the BMR is
0.75 mils this must be subtracted from all readings to get the actual DFT.
The BMR is most important when measuring thin coats or prime coats. As coating
thickness increases, not accounting for the BMR becomes less important.
Why is DFT Important?
The most obvious reason is if a contractor bids a job to give you 12 to 15 mils and you
end up with 8-10 mils, you did not get what you paid for.
Since many coatings are for corrosion protection, the main way most of these coatings
protect is by “Barrier Protection”. The thicker the barrier, the better the protection. There
are other important reasons you do not want a coating too thin or too thick.
Many coatings today cure by catalyzation. While there is some room for error built into
the coatings, some coatings require a minimum thickness to get enough catalyst to cure
properly but if the coating is too thick the reaction may occur too quickly causing other
problems including solvent entrapment and blistering..
What do I do if my DFT is out of Spec?
When discussing concerns on DFT, the coating manufacturers Technical
Representative, should be contacted. (Nothing personnel to you sales guys out there,
but unless you have confidence in your sales person, talk to the Technical Rep. and
GET IT IN WRITING). Arguments over tenths of a mil above or below spec generally
should be avoided as variations in the MBR, measurement errors, where readings were
taken or not taken can all have a greater effect then this.
Remember PA2 allows for variation of individual points to 20% below the minimum and
20% above the maximum specified range.
Type 1 meters are generally ±5% so at 10 mils there is a possible error of 0.5 mils.
Type 2 meters are generally ± 3% so at 10 mils there is a possible error of 0.3 mils.
While many electronic meters read to hundredths (0.01) of a mil at low DFT’s, it is
generally best to round to the nearest tenth (0.1) since most meter are at best ± 0.1 mil.
Once you get above 2 to 3 mils, to be perfectly honest, do not worry about numbers
after the decimal point. Round to the nearest .5 mil and that is more than sufficient for
most applications.
SECTION 7A: Measuring Coatings On Concrete (and other nonmalellic substrates)
A question I often get asked is can I measure the Dry Film Thickness of coatings
applied to concrete and other nonmetallic substrates. The answer is a definite maybe.
Using Ultrasonics, DeFelsko has developed a means of determining the thickness of
many coatings on nonmetallic substrates, assuming there is no aggregate in the
coatings
The Defelsko Positector 200 Non-destructively measures a wide variety of applications
using proven ultrasound technology. It can usually measure coating thickness over
wood, concrete, plastics, composites and more. Advanced models measure up to 3
individual layer thicknesses in a multi-layer system and features a graphic readout for
detailed analysis of the coating system. Proven non-destructive ultrasonic technique
conforms to ASTM D6132 and ISO 2808 and SSPC PA9. Limitations are coatings
cannot contain aggregate, Coating thickness is limited to 300 mils or if Polyurea about
150 when using the “D” – 0 to 300 mil probe due to the sound attenuating properties of
the Polyurea.
I am using a Positector 200 and the readings don’t make sense.
Unlike a traditional DFT meter, the ultrasonic meters require some additional setting to
get them to read properly, The meters look for the first change in speed and report this
as the coating thickness, Often this reading is much too low. There are two gates on
these meters – LOW and HIGH. Set the low gate about one half of the expected
coating thickness and the HIGH gate about twice the coating thickness. This will
generally filter out false readings but several should be taken to confirm the reading and
don’t forget to use the gel.
Also, take into account the evenness of the substrate. If you are trying to measure a 30
mil coating on an uneven concrete coating, the unevenness may be greater than the
thickness of the coating you are trying to measure. Take a minimum og 10 reading in a
small area and average them to get a good film thickness,
If readings still do not make sense there may be something in the coating preventing
proper measurements.
SECTION 8: Holiday (or as the Brits say Porosity):
Simply, holiday or porosity testing is looking for voids in the coating or coating system that go to
the substrate. For holiday testing to work requires two conditions:
1. The coating system must be nonconductive.
2. The substrate must be conductive.
Holiday testing works best on conductive metallic substrates but will also work with concrete but
is not as straight forward as metallic substrates due to variations in concrete that affect it’s
conductivity.
NOTE: There is a new method to holiday test coatings that uses UV. I will discuss it at the end
of the article.
Which Holiday Tester do I need?
A good rule of thumb is when testing nonconductive coatings on metal, coatings up to 20 mils
should be tested using a low voltage (sponge) tester. Coatings over 20 mils should be tested
using a high voltage tester. Lacking guidance from the specifications, coating manufacturer or
product data sheets, the high voltage tester should be set at 100 volts per mil of coating.
IMPORTANT: High Voltage testing can DAMAGE the coating being testing so make sure you
know what you are doing.
Holiday testing on concrete must be done using a high voltage tester. Generally, a coating on
concrete will require a higher voltage than the same coating on metal. Holiday testing concrete
is as much an art as a science and if you are not experienced, seek help to avoid problems.
I have a coating on metal less than 20 mils, what do I do?
The standards that address low voltage holiday testing are
ASTM D5162-01.
NACE RP0188.
ISO 8289A
ASTM require 67.5 volts for testing these coatings.
ISO requires 9 volts for coatings to 12 mils and 90 volts for coatings to 20 mils.
These days, with the diversity of companies doing business in the US, it is possible to be
working on specifications that reference ASTM or ISO. A low voltage sponge tester that will
handle all three voltages is the PCWI. It is easily adjustable among all three voltages; it is light
and easy to use.
What is a wetting agent and do I need one?
Many people like to use a wetting agent to reduce the surface tension of the water. If you have a
fairly thick coating near the upper end of the 20 mil range, due to the surface tension of water,
the water may bridge the void and therefore not find the holiday.
Bridging due to surface tension
If the surface is to be recoated, it is generally not advisable to use a soap as the wetting agent
because it may leave a film. The most common wetting agent is Photo-flo.
NOTEL While it does not comply with the standards, I have sued sponge testers sis by side with
high voltage testers on coatings up to about 50 mils and found all the same holidays. I would
feel very comfortable in using a sponge tester up to at least 30 mils.
Neat Little Trick.
When trying to test in a tight spot (Sponge test only), grab the sponge in one hand and with a
wet finger, feel around for holidays. Sometimes the sponge is too big to determine the location
of the pinhole. On most units it will beep and you will not feel anything. On some units such as
the Tinker and Rasor M-1, you will get a jolt similar to a static shock, but your inspector, so manup and take it.
Can I use a High Voltage testing on coatings less than 20 mils?
Yes, but the thinner the coating, the more likely the coating may be damaged by using the
improper voltage. Suggested voltages in NACE RP0188 are:
Mils
8 to 11
12 to15
16 to 20
21 to 40
56 to 80
81 t0 125
126 to 185
V
1,500
2,000
2,500
3,000
6,000
10,000
15,000
How do I do high voltage testing on nonconductive coatings on metallic substrates?
High Voltage Holiday Testing is addressed in the following standards:
ANSI/AWWA C 214-89
ANSI/AWWA C 214-91
AS 3894.1
ASTM D 4787
ASTM G 6
ASTM D 5162
ASTM G62-B
BS 1344-11
ISO 2746
ISO 2746
JIS G-3491
NACE RP0274
NACE RP0490-2001
NACE RP0188-88
The first step is to make sure you know the dry film thickness of the coating. This is the first step
in determining the voltage setting. Ideally, assuming the DFT is in spec, the specification should
tell you what voltage setting to use. If it does not, some product data sheets or application
bulletins will provide the information. Lastly, ask the coating manufacturer. If none of this works,
use the above table or use 100 volts per 1 mil of coating thickness.
An alternative to setting test voltages in the field is to use the formula developed by the
National Association of Corrosion Engineers International (NACE) and incorporated into several
Standards. The formula for thin film coatings applied to 30 mils (.76 mm) thickness is V=525
times the square root of "T" where “T” is the coating thickness in mils. Example: a coating 25
mils (.64 mm) thick would work out to an inspection voltage of 2600 volts. For thicker applied
coating the Constant changes to 1250. Example: a coating 125 mils (3.175 mm) thick would
work out to an inspection voltage of 14,000 volts.
Which is best – a Direct DC High Voltage Tester or a PULSE Tester?
There are two types of High Voltage Holiday testers – Direct DC and Pulse Types. Direct DC is
generally less expensive and works well on dry coatings. Because they put out a constant
current, as they go over the surface of the coating, they can impart a current into the coating
that will back-feed to wand causing a “false” holiday. There is a sensitivity knob on these
meters that adjusts the milliamp setting that will trigger a “beep” from the meter. For most
coatings the default setting will generally work, however if you are getting “beeps” (or Jeeps)
from the meter without a spark, you may need to change the setting.
Pulse Type Meters are more versatile in that they can be used in damp environments with
moisture condensation on the coatings. The electrical pulses are generated between 20 cps.
and 60 cps. Each electrical pulse is “on” for a time period between 20 microseconds and 200
microseconds. They do not require the sensitivity knob.
While the pulse type meter has the advantage of being able to work in damp conditions, both
types of meters will work under most circumstances with proper training.
How do I do high voltage testing on nonconductive coatings on concrete substrates?
This can get complicated and there is no easy answer. I do not have any experience in very dry,
desert like areas, but generally there is enough moisture and salts present in concrete to make
it conductive. When testing pipelines, sometimes the ground cable is just drug along on the
ground so soil and concrete both conduct electricity.
I generally recommend the following steps:
1. If available, ground the meter to rebar in the concrete.
2. If rebar is not available, often steel is bolted directly to the concrete. You can ground the
meter to the BARE steel.
3. If testing a slab with no steel, drive a piece of rebar into the ground at least the depth of the
slab and immediately next to the slab. It helps if you make sure the soil is wet. Use this as
your ground.
4. An alternative grounding method to a concrete structure is to place a 2’ X 2’ piece of
ordinary metallic window screen wire flat upon the concrete surface. Place wet sand bags
over the entire metallic surface and connect the ground wire to the screen wire. The wet
sand bags placed upon the screen wire assures intimate contact of the screen wire against
the concrete surface. This grounding method is usually sufficient for either the low voltage
“wet sponge” or high voltage “spark type” holiday detectors. Check the electrical circuit of
the detector by touching the exploratory electrode to the bare concrete substrate and
observe the audible signal. No audible signal means inadequate grounding and a better
ground must by obtained or the signal sensitivity increased.
5. As with metal, first follow the specs, next go to product data and application bulletins and
then go to the coating manufacturer.
6. If you still do not have a voltage setting, do not use the settings recommended for steel. The
best way is to find several areas representative of the coating to be tested at varying
distances from the ground. Create a holiday and determine the voltage necessary to create
a spark. If the voltage increase as you move from the ground, you need to increase the test
voltage or move the ground point.
Several coating manufacturers that make concrete coatings are now making conductive
primers. By using one of these primers, you can follow the procedures for holiday testing
metallic substrates.
Are Holiday Testers dangerous working with such high voltages?
All holiday tester output currents in the range of anywhere from 0 volts up to over 40,000 volts.
The good news is the output is in DC volts which means you cannot electrocute yourself if you
get zapped, and if you do it often enough, you will get zapped. The danger is not from the
current but that the current can make you jump. If your feet are planted firmly on the ground, this
is generally the worse damage, but if you are on a ladder or scaffold, jumping can be
dangerous. The voltage will not hurt you but the fall can.
What is with all the brushes or probes?
There are four basic types of probes:
1. Wire Brush Probe -Wire Brush probes work best on textured surfaces where the brush can
follow the contour of the surface. It comes in various sizes up to 40 inches and works well
on large surfaces. For small surfaces, the band brush that come with the 236 works well
2. Rubber Brush Probe - The Rubber Brush is a conductive rubber strip that works well on
large, smooth flat surfaces.
3. Internal Pipe Probe - Come in diameters from 1.5 inches to 12 inches for testing internal
pipe coatings.
4. External Pipe Probe -Comes in sizes from 2 inches to 36 inches for testing 360o of external
coating with one pass. Springs can be connected together for larger diameters.
I have a DC High Voltage Tester but do not understand the sensitivity knob.
In most cases leaving the sensitivity knob centered in it’s range will work for the setting. This
knob only controls the audible alarm setting and has no effect on the meter sparking or the
visual LED in the wand. On some coatings, as you run the holiday tests, the coating can
become charged and discharge back to the wand causing a false holiday indication. The
sensitivity knob is designed to tune down the audible alarm to avoid false alarms. If you are
getting audible alarms when the meter is not producing a spark, turn the knob counterclockwise
if you are getting sparks with no audible alarm, turn the know clockwise. Adjust knob as required
to find the best setting so the alarm beeps only when there is a spark.
If the knob is set full clockwise the meter will beep almost continually even when no holiday are
present.
If the know is set fully counterclockwise, you will still get a spark, but the alarm will not sound.
How do I do a holiday test with a UV light?
A couple of coating Manufactures have developed coatings that incorporates a UV pigment.
When you shine the UV light on the coating with the pigment. holidays show up as a black spot.
When you coat over the primer hand shine the light on the coating, if you see bright spots, you
have a holiday to the primer. A black (or purple) light at 400 to 450 nanometers is used to
detect holidays. You can also use UV down to 365 nm. Since manufacturers are continually
coming up with different products, make sure you know the correct frequency to test.
SECTION 9: Tape (Peel) Adhesion:
There are two basic types of adhesion tests, Peel and Tensile.
TENSILE ADHESION
PEEL ADHESION
The Tape (Peel) Adhesion Test will determine if the bond strength of a coating on a substrate is
“Generally Adequate” and was designed specifically for use on steel substrates. Use on
other substrates may not be consistent. It is not intended to determine high levels of adhesion.
Adhesion tests are DESTRUCTIVE and generally should only be conducted if a problem is
suspected.
Tape Adhesion is addressed in ASTM D 3359, “Standard Test Methods for Measuring Adhesion
by Tape Test” and ISO 2409, “Paints and varnishes - cross-cut test”.
There are two types of Tape Adhesion tests, Method A – X Cut and Method B- Cross Cut.
ASTM D 3359 provides the following guidelines:
Coating Thickness
1-2 mils
2-5 mils
>5 mils
Type Test
Method B
Method B
Method A
Spacing
1 mil
2 mils
X Cut
Under ISO 2409 for coatings 5-10 mils it allows for Method B with a 3 mil spacing.
WHEN IS ADHESION TESTING APPROPRIATE?
There are several good reasons for testing adhesion.
•
•
•
There are questions about surface preparation, intercoat cleanliness or overcoat window
times.
When you want to overcoat an existing coating
When there is a premature coatings failure and you want to determine the cause.
What Tape Should I USE?
When D3359 was developed, Permacel 99 tape was the specified tape. Permacel is no longer
made. There are several other tapes that meet the permacel specifications including the
CrossHatch Tape on our web site..
Any tape agreeable to all parties involved is acceptable but keep in mind Permacel 99 tape is
closer to drafting tape than duct tape.
When performing the ASTM tape we recommend and sell part number K0001539M001. When
performing the ISO test, the method requires ISO Tape be used (Our Part Number
K0001539M002).
How Accurate is this test?
There are many things that can affect the accuracy of the test and for that reason grading is
done on a 0 to 5 scale to avoid the appearance of greater accuracy. Some of the items that can
affect the accuracy of the test are:
•
•
•
•
•
•
•
•
•
•
•
•
Type of Tape Used
Is the coating the thickness it is supposed to be.
How well the tape adheres to the coating
How well the tape is applied to the coating
Angle of pull of the tape
Rate of Pull of the tape
Squareness of knife to the surface when scoring
o NOTE: Flexible panels need to be well supported
Sharpness of the blade
Accuracy of the spacing
Pressure applied during the cut
Age of the tape
Temperature and humidity during the test
When run properly and consistently, generally repeatable results can be achieved within one to
two units.
What do the numbers mean?
Rating
Area Removed
5
0%
4
<5%
3
5-15%
2
15-35%
1
35-65%
0
>65%
Now that you ran the tests and got numbers, what do you do with them. Part of the answer
goes back to the reason the test was run in the first place.
Make sure you know what the coating thickness IS not just what it is SUPPOSED TO BE. If you
use a 2 mm spacing on a coating reported to be 4 to 5 mils and it turns out to be 11 to 12 mils, it
will probably fail the test.
Reason 1: We suspected a problem with the application:
There is no way to give an exact answer, but for a new coating system I would expect a 4 or 5
with 0, 1 or 2 indicating a possible problem. A 3 is a little more “iffy” to judge. Was it closer to a
4 or to a 2. Is the DFT in the proper range for the test. Is the break between coats or at the
substrate.
This test is not a substitute for inspection or experience but is only one tool to be used in the
evaluation of the application.
Reason 2: We need to overcoat an existing coating, will we have a problem.
Adhesion testing is only one of the criteria used to determine if a coating can be overcoated. As
new coatings cure, they exert forces on the existing coatings. The greater the adhesiveness of
the existing coating, the more curing stresses they can adsorb without failing. Generally with
proper surface preparation, if all other factors, (condition of substrate, number of coats, current
DFT) are within reason, then after running a test patch, conditions 3, 4 and 5 are probably OK
to overcoat. If you are not sure, consult your coating supplier or a independent third party.
What is best – Single or Multi-blade?
For an X Cut you need a single blade and a straight edge. It is best if you have one that keeps
the blade perpendicular to the score line.
You can also use a single blade for the crosshatch test, however, to do it accurately requires
use of a cross-hatch template (SP3000). The advantage of this method is box cutters are
inexpensive and blades are readily available.
Multiple cut blades (TQC CC2000 or 2200) ensure proper spacing and are much easier to use.
They must be used on nonflexible, smooth surfaces to get a good cut with all blades. If all the
blades are not cutting evenly, use a piece of smoked glass, (use a candle or match to blacken
glass with soot) and lightly score through the smoke with the blade. All blades should tough the
surface or the blade should be replaced. If the blade is good, check the item being tested for
flexibility, flatness or smoothness.
Test Tips:
• Tape should be pulled at a 180o Angle.
• Make sure blades are sharp (preferably new).
• Keep blades square to the surface.
• Cut in an as even a motion as possible.
• Make sure tape is new or has been stored properly.
• Make sure surface is clean and dry before applying tape.
• Rub tape onto surface using pencil eraser.
• Pull tape within 1 to 2 minutes; remember the bond increases with time.
• Run two more confirming test in same area.
• Keep the tape as a record of the test
• It is often easier to read the tape than the panel.
• For X-cut tests use a ruler to measure failure
• For X-Cut use a straight edge.
• Report Number of Tests, Mean, Range, where failure occurred, type of test used,
environmental conditions, location of tests
• Keep a copy of ASTM D 3359 (or ISO 2409) available
SECTION 10: Tensile Adhesion
TENSILE ADHESION
PEEL ADHESION
A Tensile Adhesion Tester will determine the bond of a coating on a substrate, or cohesion of
the coating or cohesion of the substrate. Adhesion tests are DESTRUCTIVE and generally
should only be conducted if a problem is suspected.
NOT ALL TESTERS ARE EQUAL
The measured pull is highly dependent on the tester used. I.e. The psi reading taken with an
elcometer 106 will not equal the psi reading taken with an P.A.T.T.I.. There is NO valid way to
compare psi readings taken with different model testers. It is even difficult to compare readings
between different operators
Adhesion testing is addressed in ASTM D4541, “Standard Method for Pull-Off Strength of
Coatings using Portable Adhesion Testers”. It identifies 5 types of portable adhesion testers.
When choosing an Adhesion Tester, in general, the Tensile Adhesion Strength between the
different models is:
Lowest Pull:
Highest Pull:
TYPE 2 - Elcometer 106
TYPE 3 - HATE or Elcometer 108
TYPE 5 –DFD 1910, 1920, 1930, 1940, Defelsko AT-M
TYPE 4 - P.A.T.T.I. or TYPE 5 Defelsko AT-A
Unless otherwise stated, published PSI ratings for adhesion are almost always done using the
P.A.T.T.I® Meter because it gives the BEST adhesion results. Recent test I have run indicate
the defelsko AT-A tester give results close to, but not quite as high the P.A.T.T.I.
My Rule of Thumb for the Adhesion Testers based on MY personnel experiences with the
different testers:
Expected Pull Rate for 400 PSI pull with the different meters
300 PSI
400 PSI
500 PSI
600 PSI
700 PSI
800 PSI
TYPE 1
P.A.T.T.I. or AT
HATE
AT or DFD
NOTE: Many factors can affect the PSI obtained especially operator technique. The above
chart is not a hard and fast rule but only intended to be a guideline. By understanding the
different testers, you can optimize results.
MATERIALS
Coating must be on a firm substrate, such as on metal, on concrete or on stiff wood products.
Flexible or soft substrates will distort when under load and this may interfere with the bond,
reducing the adhesion. The tester needs to be supported by the sample to deliver the reactive
force. This means the choice of tester is limited by the size of the sample it can stand upon.
Small diameter pipes can be a problem due to using flat dollies on a curved surface. The HATE
and Elcometer 108 provides concave and convex dollies for pipe diameters about 8 inches to 75
inches. If using the 106 you can minimize the effects of curvature by using the support ring to
make sure the legs are set firmly and squarely. On smaller diameter pipes, below about 16
inches, adhesion will be compromised by using a flat dolly on a curved surface. As the pipe
diameter increases, the effect of the curvature becomes less.
If Adhesion tests are a specification requirement consider coating test panels in lieu of testing
the equipment. This solves the problems of testing a curved surface, prevents repairs and is
usually acceptable to the owner when it is explained to them.
Adhesion Testers
ASTM Type 2 – 106,
ASTM Type 3 - HATE, 108
ASTM Type 4 – P.A.T.T.I.
ASTM Type 5 – Defelsko AT or DFD
Type 2, 4, and 5 Testers pull on the test stub (dolly) from the outside..
Type 3 Tester pulls the dolly and delivers the reaction through a hole in the center of the dolly
along its axis and pushes the dolly off.
PULLING
There are three factors that come into play in pulling dollies”
• Manual or automatic
• Rate and Evenness of pull
• Squareness of pull
Method of pull (applying force) is applied in 3 ways.
Elcometer 106, 109: A wheel or wrench turned by hand (Manual)
P.A.T.T.I.: by gas pressure. (Automatic)
Defelsko AT, HATE or Elcometer 108, DFD 1910 1930 1940 1941: by hydraulic pressure by
hand or pump. (Manual)
Defelsko ATM by hydraulic pressure (Automatic)
Manual vs. Automatic
The rate and smoothness of the pull can affect the final tensile adhesion measured. Some
testers ensure the increase of force is at a slow uniform rate. Operators sometimes increase it in
quick steps. For manual testers, those that allow for a continuous pull rate will normally give the
highest results. Automatic tester will in general give higher more consistent rates than
manual testers.
Rate and Evenness of pull
An important aspect of pulling is to provide a direct axial (perpendicular) force. There must be
no attempt to twist the dolly off the surface, nor to lever it off. The straighter the pull, the closer
the test is to the ‘tensile’ required rather than to the easier ‘peel’. Testers where the pull head is
separated from the crank or knob, minimize twisting and levering during the pull
Squareness of Pull
To Be A “Tensile Pull”, the test fixture must be pulled perpendicular to the surface. While some
testers use ball bearings to adjust for misalignment, this does not ensure a perpendicular pull.
The only self aligning testers to date, are the DFD 1910, 1920, 1930 and the 1940.
The DFD PAT ‘legs’ adjust to be square to the dolly before doing any work. Other testers stay
perpendicular to the surface so some leverage is possible. But even when the incline is less
than 5 degrees (<1% reduction in force), some tests show a break that is off-centre, as if a
crack moved along the diameter of the dolly.
The HATE Tester avoids this problem by concentrating the force and reaction within the area of
the dolly.
RATING THE TEST
It is not enough to record the maximum stress (tensile adhesion). Standard methods require
some analysis of the material remaining on the dolly. Often the nature of the break is more
important than the pressure that it pulled.
There could be a break between layers of a coating system. One layer may be split, indicating
the cohesion is less than the adhesion. The system may break from the substrate. The cause
of this may be inadequate preparation or even dust. It is quite possible for the glue to break
between the dolly and the surface of the coating; this does not constitute an adhesion value.
Usually, most dollies have “multiple” mode failures. Record per cent adhesive, cohesive and
glue failure
Example:
Text fixture failed at 750 psi.
30% adhesive failure to substrate
40% cohesive failure in intermediate layer
30% adhesive failure between topcoat and glue.
Definitions
Adhesion Failure – defined as a failure between layers of paint or between the substrate and the
paint
Cohesive Failure – defined as a failure or break within one coat of paint.
Glue Failure – Break occurs between the glue and the pant or the glue and the dolly. This is not
an adhesion failure.
TO SCORE OR NOT TO SCORE
The specification should say whether or not to score around the dolly. ASTM D-4541 leaves it
open to the owner and the person doing the test. It cautions scoring can cause micro-cracking
that can cause lower adhesion but also acknowledges that not scoring can lead to higher pull
rates due to the influence of the adjacent coating.
The ISO 4624 and BS EN 24624 standards require a tensile test over the area of the dolly. This
means the surface must be separated from that adjacent, so a circular cutter is used around the
dolly before testing.
Generally I recommend not to score unless the coating is elastomeric, very thick or a sold
surface glued to the substrate. If the specifications do not tell you what to do, discuss with all
parties involved with the test.
WHICH METER IS BEST FOR ME?
If you are working to specifications, the specification should give both the pull rate expected and
the type of meter the test is to be done with
If cost and range are considerations, the following Table will help:
Cost
$995 - $1,200
$1,800 to $2,600
$1,800 to $2,400
$1,700 - $5,700
$5,000 - $5,200
$11,000 - $16,000
Range (psi)
0 – 3,200
0 – 10,000
0 – 10,000
0 – 17,400
0 – 11,600
0 – 23,200
Meter
Elcometer 106
Hate or Elcometer 108
Defelko AT or ATM
PATTI, 1910, 1940
DFD 1941
DFD 1920, 1930
NOTE: In the above table, some meters come in multiple ranges. Range can be modified by
switching the dolly size being used.
The most common meter in use today is the Elcometer 106, not because of it’s accuracy or it’s
repeatability, but because of its low cost, portability and ease of use. The most common meter
for testing coatings on steel is the 0-1000 psi meter for $999.00 It does require recalibration on
a regular basis to ensure accuracy. Best accuracy is in 20% to 80% of scale reading. Reported
accuracy in ASTM D4541 is about ± 40%. NOTE: This is the least accurate and lowest
pulling tester on the market.
The P.A.T.T.I. generally gives the best pull but this meter is better suited to laboratory use,
although I have used it successfully in the field. The pistons require a large footprint and a flat
area. The cost is $1,700 to $5,700 and can be ordered with piston to 10,000 psi. There is only
one size pull stub.
The HATE tester is commonly specified by companies based in Europe. It is a good choice if
you require better accuracy than given by the Elcometer 106 and is less expensive than some
of the other options costing $1,700 to $2,200 and measures up to 2,600 psi. There is only one
size dolly but you can order convex or concave dollies for testing pipe.
My new favorite is the Defelsko Automatic Adhesion tester and at $2,395 is an excellent choice
offering good cost, excellent pull rates and ease of use in the field. Testing shows pull rates
almost equivalent to the P.A.T.T.I. meter and if you are doing multiple tests, it is easy on the
hands. The manual meter works well, but it is worth upgrading to the Automatic to get the
easier pulls and more consistent results.
My old favorite is the DFD Pat Handy. While it is a Lot more expensive, at $4,524, it is light and
easy to use in the field, gives repeatable readings and comes with a self aligning head that is
separate from the crank. It works with a variety of dolly sizes and goes to over 14,000 psi. The
company is in Europe and repairs can be expensive and very slow (months). Other DFD
models are priced much to high to recommend except for C633 testing. The rig will coast
around $10,000 but is one of the few commercially available. Specified for TSA on pipe.
WHAT GLUE SHOULD I USE?
Instant gratification or 24 hours is one of the decisions to be made when choosing glue.
Generally, the most acceptable method is using a 2 part epoxy glue and allowing 24 hours of
cure time of at least 50oF for the to cure to a maximum adhesion.
There is no one glue recommended and each manufacters, and many labs, have their
favorites.
I have had good success with Ardilite Epoxy is usually used because it is compatible with most
surfaces and generally, (when mixed properly) give pull strengths in excess of 3,000 psi. It has
become difficult to find in small quantities but I still sell it.
DeFelsko, after much testing, recommends ResinLab EP11HT 2-Part Epoxy.
Loctite 907 was tested by at least one adhesion tester manufacturer and has been found to give
very good results.
Testing by Shell’s coating lab found 3M DP420 to work well for them.
TxDot likes using 3M DP 8005 and DP 8010
Sometimes it is necessary to get results the same day. It is possible to use “Superglue”
(Cyanoacrylate) on many coating types. If available use a high viscosity glue such as 3m
Pronto CA-100. Superglue should NOT be used on porous coatings because it will penetrate
the coating and can glue the dolly to the substrate. Also, Cyanoacrylates are not compatible
with some coatings such as Fluoropolymers and can dissolve, penetrate or soften coatings. For
maximum pull, generally it is best to let Cyanoacrylate cure for a minimum of 3 hours.
If you need a pull of over 3,000 psi, your best option is a heat cured epoxy. These can provide
pulls in excess of 10,000 psi without adhesive failure. Lastly for high PSI pulls, you may want to
use the coating being tested as the adhesive by putting the dolly in the coating prior to pulling.
Remember, there is no difference between putting on a top coat or gluing on a dolly. If you
would expect low adhesion such as top-coating a polyurethane with an epoxy, then the
adhesion of the epoxy glue will be effected by the same properties. If you have exceeded the
overcoat time of the coating being tested, it may be necessary to slightly abrade the surface
tested to improve adhesion. In addition, glue will not sick to oil, grease or dust any more than
paint would stick to it, so make sure the surface is clean.
CAN I AVOID DAMAGING THE SURFACE IF ADHESION TESTING IS REQUIRED?
The answer is a definite maybe. If the specification requires the coating must reach a specific
psi rating, there is no reason to pull to failure. At this point you can leave the dolly in place
which may be acceptable for some applications. (The 106 dollies are aluminum and can easily
be cut off at the neck to reduce their profile on the surface) If the dolly needs to be removed,
often a hit on the side of the dolly with a hammer will remove the dolly without pulling off the
coating. There is no guarantee this will work but the Peel force required to remove the dolly is
usually less than the tensile force.
Heat can often be used to soften the glue to make the dolly easier to remove.
Always assume the test will be destructive, and if you can remove the dolly without destroying
the surface you are one step ahead of the game.
WHAT DOES ADHESION TELL ME ABOUT THE COATING?
This is a good question without any good answers. There are many factors that affect the
longevity of coatings and Adhesion can be one of them. Adhesion tests work best to help
determine a mode of failure rather than determining how long a coating will last. Adhesion is
often more of a MARKETING TOOL than then a true indicator of how good a coating system is.
One manufacture’s product data sheet may state the Adhesion of their coating is minimum 1000
psi while another coating manufacturer states their coating system is a minimum of 500 psi. Will
the 1000 psi coating out perform the 500 psi coating? Maybe.
If the 500 psi reading was taken with an Elcometer 106 and the 1000 psi reading was taken with
a P.A.T.T.I., then the readings may actually be about the same.
If they were both taken with the same type tester, there is still no guarantee that the 1000 psi
system will out perform the 500 psi system. Adhesion is only one of many factors that may
affect a coatings longevity and is probably not a good test to use as a predictor.
HOW DO I TEST CURVED SURFACES?
There are two methods to test curved dollies. The first is to used curved dollies such as the
HATE tester uses. You can also have a machine shop put a curve on the dollies being used. It
should be noted that cured dollies only give a true tensile pull only where the dolly is
perpendicular to the curvature of the pipe. The rest of the curved surface is subject to sheer
forces so it is not a true tensile pull.
The second way is to reduce the size of the dolly to minimize the effect on the pull. Some of the
testers have multiple sizes for dollies. For flat or large curvatures, the 20 mm dolly, standard to
most testers, work fine. Manufacturers such as Defelsko have various size dollies and the
suggested dolly size is provided below:
10 mm
10000 psi
70 MPa
Min Curviture with
Minimal effect on pull
Down to 2 inches (50 mm)
14 mm
6000 psi
40 MPa
Down to 6 inches (150 mm)
20 mm*
3000 psi
20 MPa
Down to 8 inches ( 200 mm)
50 mm**
500 psi
3.5 MPa
Used mainly on Concrete
Dolly Size Max Pull-Off Pressure
Adhesion
Strength
*supplied with PosiTest AT
**requires 50mm kit
WHEN IS ADHESION TESTING APPROPRIATE?
There are several good reasons for testing adhesion.
•
•
•
There are questions about surface preparation, intercoat cleanliness or overcoat window
times.
When you want to overcoat an existing coating
When there is a premature coatings failure and you want to determine the cause.
DISTRIBUTORS FOR:
COATINGS TESTING
Defelsko
TQC
QUANIX
PCWI
ElektroPhysik
HATE
DeStearns
P.A.T.T.I.
Tinker & Rasor
TestEx
Fischer
WELD TESTING
Western Instruments
G.A.L.
Chlor*Rid
Parks Salt Meter
TQC
Oakton
Potassium Ferricyanide
PIT DEPTH
Western Instruments
DAKOTA
CONCRETE TESTING
SURFACE
PREPARATION
MBX Bristle Blaster
SSPC Standards
SALT REMOVAL
TQC
Tramex
EXTECH
Rilem Tubes
ULTRASONIC TESTING
HoldTight
FLASHLIGHTS
(EXPLOSIONPROOF)
Magnalight
SALT TESTING
DAKOTA
Defelsko
THERMAL IMAGING
EXTECH / FLIR
www.m-testco.com
281.359.2215
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