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Adhesion Problems

Why do some powder-coated parts display adhesion problems, including primer and top coat delamination?

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Q. I would like to know why we have adhesion problem on some factory rims we powder coat. We wash the rim first with an aluminum cleaner, put the rim in an oven to dry, apply a primer coat to the rims with zinc-free epoxy primer, gel primer for 7 minutes, and then apply a polyester top coat. The rim is cured after the topcoat is applied at 200°C for 25 minutes. On some rims, the primer and top coat will delaminate from the factory clear coat. –K.W.

A. After correspondence with you, we established that you are applying the primer over a wheel that already has a factory clear coat applied. We are not certain what that coating is, but it is never a good idea to apply a fresh layer of powder over an unidentified coating. The clear-coat on the wheel may be an acrylic powder coating, because that is a very common coating for alloy wheels. But it could be something else, and you cannot reliably apply the primer over any clear-coat, especially when you do not know what it is for certain. Your best option is to remove the factory coating, thoroughly clean the metal, and add a step that will ensure a good bond of the primer coat. A conversion coating and a clean blast should do the job. After that, you can apply the primer. That will provide a reliable bond. You could try roughening the clear coat with a blast media or sand paper, but that will be less reliable than removal of the factory coating and treatment of the metal.

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