Water Spots in Coating

Q. We have been having trouble with water spots showing through the cured powder film. Our powder supplier says we have a moisture problem and there is nothing wrong with the powder. We do not see the spots before coating. What do you think the cause of the spots is and what should we do about it? K.L.

A. I’m pretty sure your powder supplier is right that the powder is not the problem. You may be leaving spots from poor rinsing on the surface but you should be able to see these before coating. Or you could have something dripping on parts between the washer and the application booth. You can find this kind of spot on the surface by looking at them with an LED light before they are coated. You may be trapping moisture in seams or weld areas that bleeds out during cure and stains the part. If this seems to be the case, try pre-heating the part above the cure temperature and see if it clears up. You may need more time, air or temperature in the dry off oven.