What Causes Discoloration of Clear Coat Powder?

Coaters sometimes have problems with cloudy appearance or yellowing of the coating, and expert Rodger Talbert has a few possible solutions.


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Q: 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?

A: 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 Datapaq 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.

Q: We sometimes need to apply a second coat of powder due to minor defects such as too light a coating or surface blemishes. When we apply the second coat, we often have problems with poor appearance such as a rough, textured film, especially around the edges of the part, or inconsistent gloss. Can you offer any direction on how to apply a second coat to a part without causing these appearance problems?

A: Start by learning more about causes of the defects that force you to apply a second coat. Light coats and other appearance defects are frequently related to a lack of process controls or operator training. Accurate and consistent gun settings, a controlled environment, good maintenance, and understanding electrostatic application can help reduce defects. Operator technique and consistency are also important and often require ongoing training. Check out books or seminars on the subject, and see if you can improve your first-pass yield. Keep track of persistent problems, and work on solutions.

When it is necessary to recoat, it is important to make a few simple adjustments to the application process to get good results. Parts that are recoated must have 100 percent coverage to avoid a “dry spray” look. Parts that have been coated once will be more resistant to electrostatic attraction, so the gun will need some minor adjustment for optimum results. The ionization from the gun will rapidly build on the surface because there is no simple path to earth ground due to the insulation of the first coat. As the powder begins to build on the surface, the excess ionization will interfere with electrostatic attraction and create irregularities and texture in the film. Powder will have difficulty building near edges, and “stars” may appear in the coated surface. This is referred to as back-ionization; back-ionization occurs when the negative ions do not have a clear path to ground and they generate a repelling effect on the part surface. To overcome back-ionization, the current levels must be limited to avoid excess ions from building on the surface. The optimum current level will vary with different parts and powders, but consider a setting of less than 40 microamps—it may be 20 microamps or lower.

Typical adjustments for recoating include a reduction of current levels, a slightly farther gun-to-target distance and a slight increase in flow rates. Of course, this is easier if the recoats are segregated and coated in batches. Often the recoat parts are on the line with raw parts. In this case, it may be necessary to use one setting that works well for both.

Q: We recently ran a batch of parts that were coated black but were loaded with white specks when they came out of the cure oven. We immediately suspected cross-contamination of white powder in our booth or spray guns, but our line operators insisted that the problem was from white contamination in the powder material. We cleaned everything and put a fresh box of powder on the line, and the problem was improved, but we still saw some specks on some of the parts. For now, we are getting by, but we are losing a lot of parts and need help to troubleshoot the source of the problem.

A: The first step is to take the questionable powder and spray test panels in a different facility or line so you can be sure the powder is clean. But this may already be unnecessary, since you have changed boxes and still have the problem. Still, spray up a few panels to confirm that the powder is not the source. If they are sprayed somewhere else and come out clean, you know the contamination is in your system.

From what you describe, you almost certainly have a source of white powder in your system that you have not found. You have cleaned the delivery system (guns, hoses, pumps, etc.) and booth interior, so you need to dig deeper. I would start with the feed hopper. Check the fluid-tile and gasket area. Powder can get trapped in the flange area if the tile is worn or damaged, or the flanges are not mating properly. Check the pumps very carefully to make sure they are in good repair and perfectly clean. Look in vent hoses and transfer pumps to make sure they are clean, too. If you use a sieve, make sure it is clean and the screen is intact and properly installed. Check out all areas in and around the booth for a source of white powder that is built up and being entrained in the system. In other words, do a much deeper dive in cleaning the hidden areas of the system than you already have to find the source.

Q: We are a building contractor that works on a variety of structures and accessories for buildings all over the world. We recently installed some powder coated aluminum gutters and downpipes as well as powder coated mild steel handrails to a building situated in an exposed marine environment. The items have been exposed to the elements for approximately 18 months, and the powder coating has begun to fade and chip in some areas. Can you give us any advice or information as to why the coating may have broken down in such a short time of exposure?

A: When you say the parts are “powder coated” you fail to provide the essential facts that are needed to answer your question. How were the parts pretreated? What kind of powder was used? Was a coat of primer applied, and if so, what kind of primer was used? How many layers of powder were applied, and how thick was the final layer? What kind of testing was done to ensure that the coating was within the needed performance range?

I will speculate on what probably happened: There was no written specification on what to use or what the performance requirements were. The powder coater was given a color and followed its own methods without direction on what was needed. The coater did an inadequate job of treating the metal, used the wrong powder, did not apply it in the correct thickness and did not test the performance.

It is important to understand that powder coating is a process that is made up of several discreet steps that are designed to deliver a specified outcome, and the final result should be confirmed by testing. That is the only way to deliver the needed performance.

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