Spot Weld Marks
Q. I’m the plating and paint shop foreman of a company that makes office machines. In order to get a first-rate finish, we use a five-stage washer to apply an iron phosphate. After dry-off, we apply an epoxy primer and then a polyurethane topcoat to all painted parts with an electrostatic spray gun. Total dry film thickness is 3 mils. On certain sheet metal parts made of 16-gauge cold rolled steel, we have trouble with spot weld marks showing through the paint finish. How do we eliminate this problem? B.F.
A. It is often difficult to hide surface defects with a paint film, even at a 3-mil dry film thickness. Paints, as well as powder coatings, tend to conform to the surface on which they are applied. Paints over spot weld marks, which come in two forms—holes and ridges—will follow the rules of surface coatings. These surface defects can only be eliminated by cosmetic repair, requiring hand work. I suggest you fill the holes using a body putty or filling compound and then sanding them smooth. If the weld marks are ridges, they can be removed by hand-grinding using a power tool.
Powder coating is one of the most durable finishes that can be applied to industrial manufactured products, and offers excellent corrosion protection and is very safe because of its lack of volatile organic compounds.
Metal fabricators that laser-cut with oxygen take steps to prepare parts better for powder coating.
What you should know about cleaning and electrocleaning.