Primer is an undercoat for paint that improves the adhesion and appearance of the painted surface. Primer helps to fill thirsty new wood so that fewer coats of paint are required. Primer is useful for hiding stains and irregular coloration of new wood. Primer with stain-blocker helps to prevent the leaching of sap, tannin and other colorants from the wood through the paint surface. Below are some guidelines for the appropriate of use of primer on various surfaces.
New Wood - Prime with a latex or oil-based wood primer. When painting a variety of wood that tends to stain, such as pine, use a "stain-blocking" primer. Latex or oil primer can be used for stain-blocking, but oil tends to be more effective.
Old Wood, Unfinished - The same advice applies as for new wood except that the surface must be properly prepared before priming. Thoroughly sand weathered wood to remove loose particles and splinters. Clean the surface to ensure proper adhesion.
Old Wood, Painted - If bare wood is exposed, if the old paint is chalky or glossy, then a primer coat should be used. Priming the entire surface will improve paint adhesion and appearance. If the existing paint is adhering well and not chalky, then primer may not be necessary.
Masonry - Primer helps to fill porous masonry surfaces thus requiring fewer coats of paints. Primer also helps to reduce the texture from telegraphing through to the paint layer. Some very porous surfaces benefit from the use of a sealer.
Metal, Bare - Bare metal may have a coat of oil which should be removed before applying primer. Primer is important for the proper adhesion of the paint coat.
Metal, Oxidized - Oxidized metal should be sanded to remove as much rust as possible. Use a non-metallic scouring pad to clean and remove any particles.
Metal, Painted - Material that is rust-free and has no exposed metal generally does not require a primer coat.
