Cloudy Clear CoatsWe are a job shop that just took on a new job that requires us to apply a powder clear coat over polished cold rolled steel. We have never worked with clear coat powders before. Sometimes the parts come out with brown lines in a wavy looking pattern. The coating looks cloudy and the brown color appears along the wave pattern and the edges of the part. We have been told that the problem is in the cure oven but if that is true why do some parts come out good and some come out bad? We use a five-stage iron phosphate system for pretreatment and our supplier tells us there is no problem in the washer. The parts appear clean coming out of the washer and there is no oil or other discoloration on them prior to coating application. The coating is applied by manual spray guns. We have checked our compressed air supply and there is no oil or moisture in the lines. Any suggestions?
While there could be several causes of discoloration in a clear coat application, the most likely cause of your problem is too much film build. It is easy to apply too much film with clear coat powder materials because the operator cannot see the coverage like they can with a color. They may tend to apply the material too heavy because they are trying to "hide" the steel. A clear that is applied too heavy will sag in the oven and form the wavy pattern that you describe. The thick film will have a hazy look to it and may discolor in areas with especially heavy coating, like in the areas where it sags or along edges. Check your film build and work on the application of the correct thickness.
Conversion Coating Complicated?We run a batch powder coating operation using a blast cleaner to remove dirt, oil and old paint from parts before we coat them. Some of our customers have asked us to apply a conversion coating for corrosion resistance. How complicated is it to apply a phosphate coating. Can it be done manually? Does it really help prevent corrosion?
Conversion coatings create a surface condition that helps promote good adhesion and retard the spread of corrosion. Iron phosphate coatings can be applied by dip or a spray system. It is somewhat more complicated than a manual blast operation because you must control the chemical solutions. You also need to invest in tanks or a spray wand. It works very well on light gauge parts that need to be cleaned and degreased and may not hold up to the pressure of a blast operation. Iron phosphate helps promote coating adhesion but does very little to inhibit corrosion. Some non-chrome seal rinse products can be very helpful but they may not work in a batch operation and they may be material specific. For extreme corrosion resistance you need zinc phosphate or chrome, processes that would be much more costly and complicated, not to mention the environmental concerns associated with these materials. If the powder is applied properly with sufficient thickness and complete coverage the adhesion over the blasted surface is very good and the corrosion protection from the powder film is very good. If a customer insists that you add corrosion protection with a conversion coating I suggest that you look for a source that provides zinc phosphating as a service and have them clean and phosphate it for you. Bring it back to your shop and coat it as soon as possible so that it does not have a chance to pick up any moisture or contamination.