A primer is a special type of paint that goes on before the finish coat of paint. Primers lay the foundation of your paint job and can be used on wood, metal, drywall and concrete. Whether you're painting interior or exterior surfaces, primers ensure that the painting surface has an ideal, uniform texture (slightly coarse) so that paint adheres effectively. In addition, primers seal up porous surfaces and prevent stains and previous colors from showing up underneath your paint job. They also help even out walls after you've spot patched and made repairs.
While paint can be applied to already-painted surfaces without priming, it's usually better to prime so that your work is as durable and long lasting as it can be. When working on painted surfaces, priming is necessary if you're switching paint types. For example, going from oil-based paints to latex-based paints and vice versa, or changing colors drastically. Also, if paint is cracking, crumbling or if there is surface damage, priming can eliminate any problems with your paint job (after you've made repairs and prepared the surface).
Primers are formulated for interior surfaces, exterior surfaces, metals and in particular tints. Interior primer seals, increases adhesion and creates a uniform surface for walls, etc. Exterior primers minimize cracking and mildew growth, and protect masonry surfaces from alkalinity and efflorescence. Exterior primers come in specific formulas for use on wood, masonry or metal. Metal primers provide a tight bond between the surface and topcoat and inhibit corrosion. Tinted primers improve the end result of your painting project; they work with the finish coat color to boost color accents for a better-looking job.