Seeds in the Paint Film

Q. We have a coating line in our shop. For one of our customers, we recondition equipment cabinets. After reconditioning, they are stripped and repainted. In our shop, we paint a variety of materials such as steel aluminum and plastics. After stripping, the cabinets are filled and sanded to smooth any surface defects. After that they are primed and topcoated. When we spray dark colored topcoats, we often find defects in the form of seeds on the surface. This doesn’t seem to happen when we spray light colors. Another problem we have from time to time is dust. What must we do to get seed-free and dust-free paint jobs? S. L.

A. The problem of dust and seeds in your paint finish may be related. We usually associate seeding with agglomeration of pigments in the paint. Black and dark color paints which contain finely ground pigments are more likely to seed because of pigment agglomeration. These seeds can be removed by filtering the paint. It is also important to note that dust can act as a nucleus for seeding in paints. Seeding can be caused by dust on the parts to be painted or by airborne dust settling on wet paint. This latter condition is the result of grinding and sanding near the painting area because the suction or draft caused by the spray booth exhaust fan will draw in dust along with the makeup air.

For a quick fix, good house keeping by removal of dust from floors, walls and other surfaces will also help. Immediately before painting, wiping the part with a tack rag will also help. Good painting practice dictates completely isolating the finishing area from the sanding area and to filter the spray booth makeup air. This calls for enclosing the spray booth.