We are having some issues with application of powder on zinc castings. We use the same five-stage washer that we use for steel parts, which cleans and then applies an iron phosphate conversion coating. Our chemical suppliertells us this should also work for these zinc parts, but we keep getting adhesion failures and bubbles in the powder coating. We are wondering if iron phosphate is effective for zinc, or are there other options for zinc treatment? It sounds like you are seeing outgassing due to porosity in the casting. A cast part has trapped pockets of air in the body of the casting that may cause blemishes in the coating during the cure cycle. Air that is trapped near the surface may expand and rupture the film when the part is heated. There are several ways to help mitigate the issue: 1) Preheat the part to drive off some of the trapped air that causes the problem; heat it to a temperature around 50°F greater than the ultimate cure temperature, cool it down and then apply the coating. 2) Cure at the lowest temperature possible within guidelines supplied by the Technical Data Sheet for the powder to limit the problem. 3) Use a powder that is formulated for a flow cycle that will help release air without leaving a blemish and limit the impact of outgassing.
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