Marks on Recoated Parts

Q. We get some blemishes on parts when we recoat them that are sometimes referred to as back-ionization and in some cases pinholes through the coating. How can we avoid these marks on the recoated parts? How can we differentiate between back-ionization and pin holes? A.B.

 

A. Back-ionization occurs when the current level on the part surface is too high and an excess number of ions build up under the film as it builds on the part surface. The excess ionization creates a corona field under the film and generates positive charge that discharges through the film toward the spray gun electrode. It can cause excess orange peel, star-shaped ruptures in the cured film or pinholes if it is severe. The way to control it is to manage the level of current flow to the part.

There are a couple of good ways to control the current draw to the part surface. Use a current limiting circuit on the gun controller if you have one. Many newer spray guns have a controller that limits current or a factory pre-set for recoat parts. Move the gun farther from the target. Pull the gun back one or two inches and increase the powder flow a little to compensate for the greater target distance. If you are using reclaimed powder be sure that the grind size is not too fine. In a cartridge recovery system the fines can accumulate and cause application problems. Avoid excess film thickness. A thicker film means that there will be more ions in the film and under the film and greater resistance to further deposition. Control gun current level and make sure you have good earth ground to avoid these defects.