What Causes Discoloration of Clear Coat Powder?

Question: What Causes The Cloudy Appearance or Yellowing of the Clear Coat Powder?

Details: We occasionally get a job that requires the application of a clear coat powder over a color, but we sometimes have problems with a cloudy appearance or yellowing of the coating. What can cause this?

Answer: Yellowing or a milky appearance of a clear coat powder can be caused by the amount of film applied or the cure oven.

The first thing to do is to check that the film meets the manufacturer's specifications. It is not unusual for operators to apply too much clear coat, because they are used to looking for opacity and expect the surface to look white. The thick film does not provide the clarity that is needed from the clear.

If the film thickness is correct, the problem could be related to over-cure of the powder or oven-fouling. Run a temperature recorder or similar device, and make sure that the time and temperature are within the recommended range. If the film thickness and cure cycle are accurate, the final possible cause is inadequate exhaust volumes. If the exhaust rate is too low, the oven will build up with gases that can cause the discoloration of the coating. Have an oven manufacturer or consultant check the exhaust and make sure it is within the recommended range.

Pink Discoloration of Clear Powder Coated Part

Question: What could cause a pink discoloration after a part is cured?

Details: In order to powder coat a grey nylon furniture part, I preheat the part, powder coat it with a white coating, cure the white coating, and then apply a powder clear coat over the white and cure that. The issue I have is that when the part comes out of the oven it appears pink. I have applied the white and the clear coatings on other nylon parts from another company, but they don’t come out pink. What could be the cause of pinking in this scenario? J.S.

Answer: Nylon is a thermoplastic material that can be affected by heat. I suspect the heat of the oven is causing some breakdown of the nylon during cure and that is creating the discoloration. There is a difference between the composition of the nylon that is showing the pink color and the other parts you have sampled. See if you can find out something about the nylon composition and how the two different products compare. Also, check film thickness and make sure you are getting enough to hide the substrate.

Brown Spots After Clear Powder Coat has Cured

Question: Once the part comes out of the oven, there are brown spots in the film. What could cause this?

Details: 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.

Answer: 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.

Rodger Talbert

Rodger Talbert has been working in liquid and powder coatings for over 40 years, designing and improving coating systems and process. His work experience includes complete system concepts for large and small companies such as Caterpillar, John Deere, Steelcase, the US Navy and many others. He also has done many projects to improve efficiency, solve problems and train workers in small shops and big corporations.