A primer is a preparatory coating put on materials before painting. Priming ensures better adhesion of paint to the surface, increases paint durability, and provides additional protection for the material being painted. Paint colors, dyes and compositions are getting better all the time. Effects that would have been unthinkable just a generation ago are becoming easier than ever to achieve, from special matte finishes that resemble suede to effortless crackle glazes that make faux effects a cinch. One thing paint cannot do terribly well on its own, however, is bond to every possible surface without a little help. This is where paint primer comes in.
The basic rule of thumb for paint primer is that it is necessary whenever you cannot create a totally flat surface for painting. Surfaces with cracks, holes or depressions that aren’t properly sealed could greatly reduce the paint’s ability to set effectively, so inherently porous substrates such as wood, drywall and concrete tend to come with primer recommendations attached.
Usage of primer:
Primer is a paint product that allows finishing paint to adhere much better than if it were used alone. For this purpose, primer is designed to adhere to surfaces and to form a binding layer that is better prepared to receive the paint. Because primers do not need to be engineered to have durable, finished surfaces, they can instead be engineered to have more aggressive filling and binding properties with the material underneath.
Primers can usually be tinted to a close match with the color of the finishing paint. If the finishing paint is a deep color, tinting the primer can reduce the number of layers of finishing paint that are necessary for good uniformity across the painted surface.
Primer is essential for the following reasons:

  1. Priming seals the surface, assuring a smooth base for your topcoat of paint. Because primer is formulated to seal the surface, its application will make it easier to apply the topcoat. Primer seals the base, preventing uneven absorption, which leads to uneven paint color.
  2. Priming maximizes topcoat hide, so it's ideal for color changes. Want to go from cranberry red to sunshine yellow? Even two or three coats of quality paint may not completely hide your previous color, whether it's lighter or darker than your new choice.
  3. Priming increases paint coverage and topcoat adhesion. Many household surfaces are either too porous or too slick to allow two coats of paint to adhere properly. Porous surfaces like wood, drywall and masonry soak up paint unevenly and require multiple coats to achieve the desired look. Slick surfaces such as glass block and ceramic tile, and even previously painted glossy surfaces such as cabinets and furniture could cause the paint to flake if not primed first.
  4. Priming promotes a consistent sheen and uniform finish. Once you have chosen the paint sheen (flat, satin, semi-gloss, high gloss) that best meets your needs, prime the area first to ensure that the topcoat is uniform and will stay true to the finish you have chosen.
  5. Priming seals stains from mold and water damage so they don't bleed through your new paint. Bathrooms, basements and other moisture-prone areas are liable to have mildew growth.

Primers are available in oil and water-based formulas, for interior and/or exterior applications.

Summary of paint primer: Many paint primers for wood have lots of dense pigments in them that act as microscopic liquid-spackling compound. The pigments fill in all of the low spots in the wood so the finish paint will look like satin.
Modern primers are often water-based and dry very rapidly. Some can be recoated with finish paint in less than an hour. Using a primer will actually speed up the job, not slow you down.
Primer used for different kinds such as: a).Metal b).Plastic c).wood and it is necessary to cover water stains, smoke damage, mold and mildew, fresh drywall, dark colors (if covering with a light color), weird types of paint such as blackboard paint, rust stains, crayon, marker, pet marking stains, oily stains from items such as air freshener plug-ins, painting metal, etc,.