Adhesion Testing

Q. We have our 29-gauge hot dipped galvanized steel roll coated at the mill. They apply a 4-mil dry film thickness plastisol finish. We test the adhesion of the plastisol to the steel on every incoming coil by crosshatch cutting through the plastisol and galvanizing all the way to the steel substrate. The crosshatches are cut at 1/16 inches on a side. Next, we perform a 70-inch-lb reverse impact test on the crosshatched areas. Finally, if we are able to remove any of the finish after the impact test using a sharp instrument, we consider adhesion to be non-acceptable.

Recently, we have gotten a broad range of results which is causing us some concern. Our supplier claims this kind of test for adhesion is too tough and inconclusive relative to the decision as to whether good film adhesion exists or not. We insist that the test has been perfectly reliable in past years and it should be now. Is this test reliable for paint film adhesion? S.S.

A. I agree with your supplier that your test is too rough and may be inconclusive. The phrase in your test description that concerns me is “using a sharp instrument.” I assure you that even a small pocket knife will remove the most durable coating.