Brown Spots in Clear-coat

Q. We have been applying powder to many different parts for the past 10 years. We have a new project that requires the application of a clear coat epoxy powder over CRS, and we have never done any work with clear powder. The parts are pretreated in a five-stage iron phosphate washer that uses an alkaline cleaner in stage one.

The parts look good coming out of the washer and there are no obvious stains. The clear coat is applied in an automated booth and it looks fine before it is cured, but when it comes out of the oven, there are brown spots in the film that look like drips of oil. We do not think there is any contamination on the part. We have inspected the oven interior but we cannot find any source of contamination there.  What would cause this? J.B.

 

A. You do not mention the film thickness of your coating, but it sounds like you are applying too much coating. This is easy to do with clear coats because it is harder to understand that some metal still may be showing when the coating is applied at the correct thickness.

Operators used to colors want to hide the metal, and they may be putting on too much. The coating sags a little from too much film and the edges of the sag turn brownish in color. Check the film thickness and make sure you are within the manufacturers recommended range. Note whether the brown spots run in the direction that the part is hung, an indication of sagging from gravitational pull.
You should also check your exhaust rate. If the exhaust rate is too low, the oven can become saturated with gases and that can cause yellowing of parts, although it usually around the edges and not like oil drips, as you describe.