A generic sealer performs a similar task in linking the topcoat to the substrate, but not the same task. A sealer, with its high degree of resin, seals the porosity of the substrate. Without it, the substrate would suck up the finish coat like the ground sucks up rainwater. Frank Granelli, director of marketing for William Zinsser and Co., a manufacturer of primers and sealers, expanded on Hardy's definition. As far as painting goes, primers are used in preparing surfaces for painting, and they differ from paint in the resins they contain, and the amount of resins.
Primers usually have higher resin content than paint.
The only time a primer gets a high PVC and low resin is when it's designed to hide something, for example, a drywall primer designed to hide tape. If, on the other hand, you want to hide profile, if you want something to give you a high penetration, soak down into the wood or whatever surface you're painting, pigment just gets in the way. In general, in sealers, the resins are asked to do a little different job than they are in primers. Usually, sealers are asked to be a good hold-out material, material that will give you a good gloss when you're putting on a glossy topcoat. The flatter the primer, the more likely the gloss of the top coat will be sucked down into the primer.