Powder on Galvaneal

Q. We have customers that want a smooth powder coating on a galvaneal finish. We prep with a light alcohol wipe, prebake the parts to allow for off-gassing, then spray with a smooth powder. We are having problems with “craters” and specs, so the surfaces won’t pass a class “A” test. We have used the same powder on raw metal parts that were either iron-phosphate coated (steel) or have a clear chemical conversion coating (aluminum) and have not had these issues. Is there a way to coat a galvaneal part with a smooth powder that does not have a special additive? Would using a lower-cure temperature (but longer time) help? The issue does not occur when using a textured powder—only a smooth. A.B.

A. Galvaneal is an alloy that uses zinc to modify the steel surface. Like any alloy, it can have some irregularities at the surface that may cause problems with an organic finish. Your method of preparation is not very effective. Alcohol wiping will smear dirt around but not really remove it. Pre-baking will drive out some trapped porosity and burn off some impurities but not really clean the part. You need something that will better clean the surface to remove any impurities: a heated spray washing process, followed by a good rinse and possibly a conversion coating. Galvaneal does not usually outgas severely like a casting, but you may need a powder formulated for more flow to avoid craters from the release of trapped air. Your pre-bake should be at least 50°F above your cure and, yes, a longer cycle at lower temperature is a very good idea. Try a better cleaning method, go ahead with your pre-bake and cure at a lower temperature.