Q. We are painting a plastic part using a two-coat system. The problem is that we have a lot of scrap due to what looks like dust in the paint film. At least it appears to be dust. Is there some process or procedure that we can use to determine exactly what kind of dust we have in the paint film, such as metal, fiber, etc.? Perhaps it is not dust at all but something else. Can you help us? J.N.
A. It is the policy of Painting Clinic to withhold names of paint and equipment manufacturers, but I can tell you that unlike the common spray booth, the painting equipment you are using is designed to apply coating in a dust-free enclosure. If it is functioning correctly and you are doing everything right, there should be no dust in the paint film.
Assuming your equipment is functioning correctly, your film defect could be caused by particles already on the parts as they are entering the painting machine. Plastic parts are known to attract particulate matter when they acquire a static charge on their surfaces. The defect could also be caused by microblisters or bubbles in the paint film. Agglomerated pigment particles will also cause this defect and they are often very large. Another possible cause is dirt in the paint.
There are several things you can do to identify the offending particles:
1) Examine the film under a microscope.
2) Filter a wet sample and examine the filtrate.
3) If you don’t have lab equipment, send wet samples and dry films to a testing lab.
4) Ask your paint supplier to examine the samples.
To help eliminate these film defects, I recommend removing dirt and pigment agglomerates by filtering paints before pouring them into painting equipment; using and maintaining in-line paint filters; and examining parts surfaces for particulates before painting. If necessary, the static charge on plastic parts should be discharged.