One of the keys to success on your commercial painting job is proper paint adhesion. Therefore, an adhesion test is important to ensure job satisfaction. There are many recognized ways to determine how well a coating has bonded to the surface.
The infamous “thumbnail test” is NOT a recognized method.
All too often, I hear comments like, “The coating is peeling off when I dig my thumbnail into it.”
Coatings must have sufficient time to dry and cure prior to using a standard test method for adhesion.
Adhesion Test Methods
X-cut tape test
Use the X-cut tape test at job sites. Do it like this:
- Use a sharp blade. Then, two cuts are made into the coating down to the substrate to form an X.
- Place masking tape over the X-cut and then remove rapidly.
- The X-cut area is then inspected for removal of the coating from the substrate or previous coating and rated.
Cross-hatch tape test
The cross-hatch tape test is typically for use on coatings less than 5 mils thick.
- Using a sharp blade, cut a cross-hatch pattern rather than the X pattern.
- Place masking tape over the cuts and then remove rapidly.
- After the tape has been pulled off, the cut is then inspected and rated.
A standard method for the application and performance of these tests is available in ASTM D3359.
Pull-off test
A more quantitative test for adhesion is the pull-off test. Attach a loading fixture, commonly called a dolly or stub, by an adhesive to the coating. By use of a portable pull-off adhesion tester, a load is increasingly applied to the surface until the dolly is pulled off.
The force required to pull the dolly off yields the tensile strength in pounds per square inch (psi) or mega Pascals (Mpa). Failure will occur along the weakest point in the system comprised of the dolly, adhesive, coating system and substrate.
This test method maximizes the tensile stress as compared to the shear stress applied by other tests and the results may not be comparable. A standard method for the application and performance of this test is available in ASTM D4541.
Adhesion Test: When in doubt
If I were unsure about the substrate I’m painting, I would follow this process to ensure good results:
- After preparing the surface, apply a test area of a high adhesion primer. (Because of the exceptional adhesion of these products, sanding may not be necessary for most clean, paintable surfaces.)
- Allow the primer to dry properly and test for adhesion, using the X-cut tape test.
So Amigos, do you recommend any other methods to test for adhesion? Share below…
Also, check out Paint Amigo’s taping product recommendations below (Amazon affiliate links)…
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