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Ask the Expert Question-and-Answer Archive

by Ron Joseph

October, 2006

Adhesion of Elastomeric Paint to Stucco

Q: How can one tell if an elastomeric paint was used? I am painting the stucco panels on a Tudor and do not want to apply a coating that will have adhesion problems. The panels are in good shape with no visible peeling or cracking. I have no idea how long it has been since the last painting.

A: The common method for conducting an adhesion test its to perform ASTM D-3359 Method B, which is a cross-hatch adhesion tape test. In this procedure you use a special knife with 11 closely spaced blades and scribe two lines in the paint; one horizontal, the other vertical. Depending on the thickness of the paint ASTM gives you a choice of the spacing between the knife blades, but for purposes of this answer assume it to be 1 mm. After you have scribed the two lines one over the other, you will have a grid comprising 100 squares. You now apply a piece of special adhesive tape, much like masking tape, over the grid. Apply pressure onto the tape to get good adhesion between the tape and the coating and then peel the tape off at 180 degrees. Adhesion is measured by evaluating the number of squares that have come away from the painted surface and adhered to the back of the tape. Clearly, the more squares that come away, the poorer the adhesion.

In the case of stucco this test is much more difficult to perform because of the profile of the stucco. If you can find areas in which the old paint was applied over a flat, non-textured surface, you can still conduct the ASTM test.

A cruder test is to simply scribe a large "X" into the coating and try to loosen the paint in the apex of the "X". If the paint holds firm
this is an indication of good adhesion, while if the paint in the apex pulls away like a sheet, you clearly have poor adhesion.

While these tests are strictly qualitative, there is no guarantee that you are getting good adhesion, even if the paint passes the cross-hatch test. The more tests you perform that have positive results, the more likely it is that you will have good adhesion.

Best wishes,

Ron Joseph

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