Primer 101
You might think of primer as just "that stuff beneath the paint," but the undercoats serve several important purposes critical to a good paint job, and there are a variety of undercoat types. First, primer protects the metal from corrosion. Second, it provides the final level of bodywork, filling in minor imperfections in a car's surface. Third, it seals the prep work below, and fourth, it provides a uniform surface for the paint to adhere. These are the types of primer and what they do:
*Self-Etching Primers/Epoxy Primers provide adhesion to the metal surface and all products that follow and also corrosion resistance. If you take a car down to bare metal, one of these should be used. This primer is generally not sanded.
*Primer Surfacers fill small scratches and imperfections in the metal surface. They dry quickly and sand easily-in fact, several layers are often applied and sanded until a satisfactory level of straightness is achieved. Some primer/surfacers have direct-to-metal etching qualities-read the tech sheet to be sure.
*Primer Sealers seal the preparation layers below and provide a uniform bonding layer for the finish coats. They can be tinted for color-matching purposes and are not sanded as long as the paint is applied within the curing window. If primer sealer is allowed to fully cure, some may require sanding before painting; again, follow the manufacturer's recommendations.
Normally, you'd think the smoother a surface is sanded the better. Not so, says our man Prewitt. We'll cover this more elsewhere, but paint products make two types of bonds: chemical and physical. If you sand primer too smooth, the next layer can't make the necessary physical bond. "Under a microscope, you see little teeth sticking up," he tells us. "That makes the physical bond." The bottom line is if you sand primer too smooth, the following layers can't stick. In a nutshell, 400-grit is as fine as you need to get during the final wet-sanding of the primer surfacer.