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Coating Adhesion Failure

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Q. I have an issue with my RAL 5008 mini tex not adhering to aluminum extrusion. The coating bubbles up, but if I send it through the oven a second time it lays down and looks great. It even passes a crosshatch adhesion test after a second cure cycle. We use an iron phosphate wash with acid content between 3 and 3.5 percent, and in tanks 2 and 3 we run our pH between 5.5 and 7.–J.L.

A. Iron phosphate is not a particularly good treatment for aluminum. It can help remove organic soil, but is not a true conversion coating for aluminum. Without a conversion coating, the aluminum will oxidize rapidly, and that will interfere with adhesion. You should conduct a wet adhesion test as described in AAMA 2604 after your second bake. It is likely that you will fail this test. A dry-in-place, non-chrome seal rinse for aluminum should solve your problem.

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