White Spots in the Paint

Question:

I am doing mass-painting of car bumpers made of PP resin material using a robot. The paint I’m using is high-solids. The problem is when the robot sprays the primer, I get a few white spots on the surface. Under microscopic view, they looks like lumps in the primer itself. These lumps cannot be felt feel with the finger. But when oversprayed with topcoat, the white spot seem to cause what looks like dust on the surface. After clearcoating, the spots sometimes look like fisheyes. Can you suggest what I should do to eliminate the white spots? I. S.

Answer:

Anything that does not belong in or on a paint film is a contaminant. Contaminants can be present in the paint itself, come from the painting equipment or be drawn into the paint spray booth with the make-up air. Contaminants in the paint can be dirt or other foreign materials, pigment agglomerates or resin gel particles. These can be eliminated from the paint by filtering when filling the painting equipment reservoirs and by using in-line filters in the painting equipment. In air atomized painting equipment, contaminants can also come from the compressed air. These can be removed by filters and traps in the air lines.

Airborne contaminants in the factory air can be drawn into spray booths and settle on the surface of painted parts. These can be eliminated by filtering spray booth make-up air. It is well known that airborne particles can travel long distances in factory air. Other sources of contaminants include baking ovens, conveyor chains, hangers and hooks. Conveyor chains must be cleaned. When hooks and hangers are cleaned in burn-off ovens, the powdery residue, if not completely removed, can fall onto the surface of painted parts. I recommend you determine the exact composition of the white spots and then find their source so they can be eliminated.