“What is the best out-gassing primer to use? I have a zinc casting that is about a quarter of an inch thick and is used in laboratory work. This part gets a smooth epoxy powder coat finish. We have tried pre-baking and several out-gassing primers. We have varied the coating thicknesS. Is zinc or iron phosphate the best pretreatment for this part?
I’ve been scratching my head for about five years trying to figure this one out. We are currently using a wet primer. Out of 160 parts, about 40 end up with defective finishes. I hope you can help us. Thanks.”
“Zinc die castings are notorious for out-gassing when subjected to the temperatures required to cure powder coatings. The amount of out-gassing depends upon the metallurgical composition, casting vents, chill locations, speed of “pour”, etc.
Sometimes preheating the part can reduce the problem, but not always. Out-gassing primers are becoming popular to reduce this problem. You have tried both of these approaches with some success.
Next you should ask your powder supplier for an out-gassing topcoat powder formula. During cure, these materials stay in the liquid state for a prolonged period of time. This allows any gas to escape. The escaping gas breaches the coating and forms holes. However because the coating has been formulated to stay in a liquid phase longer, the coating flows back over these holes to provide a smooth surface.
If the casting has a large amount of entrapped gasses, then this approach will provide limited benefits if all the gas has not fully escaped because eventually the coating will have to gel. It may be that you will not ever entirely eliminate this problem, however using all the recommended methods may reduce your defect rates to acceptable levels”