Primer (paint)
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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.
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[edit] When to use primers
In general, a primer is necessary when painting any kind of porous materials, such as concrete and especially wood (see detailed description below). Priming is mandatory if painted material is not water resistant and will be exposed to the elements. Priming gypsum board (drywall) is also standard practice when building a new home or building because it seals the wall and aids in preventing mold. Priming is also a good idea if material is dirty and for some reason cannot be cleaned, or when painting light colors over existing dark colors. Primer and paint form a chemical reaction that allows the paint to adhere much better than if paint were used alone.
[edit] Primers for wood
There are several reasons why using a primer before painting wood is mandatory. First of all, wood is very porous and will absorb moisture from paint, drying the paint prematurely. As some paints undergo chemical reactions during the process of curing (for example, latex-based paints actually polymerize when curing), they depend on water or solvent being evaporated, not being absorbed. A layer of primer will help the paint to undergo proper complete curing cycle. Second, unless you are painting wood to a much darker color, if you don't use primer, several layers of paint (as many as 8) will be needed to completely obscure the wood grain, and ensure even color. In general, primers are anywhere from 10% to 75% cheaper than paint, and using a primer prior to painting simply makes sense economically. Getting an even color is also a concern, as wood is not uniform, and will absorb paint differently. If you want for the wood grain to show, you really would be better off using wood stain, not paint. And lastly, if wood is exposed to moisture, a thin layer of paint will still be water permeable. The end result will be warped parts, mildew, and dry rot (a misnomer, because dry rot only exists in the presence of water, but it's a common name nowadays). Thus, make sure to use a primer every time wood is exposed to the elements.
[edit] Primers for metal
Some metals require a primer (such as un-treated aluminum) while others may not. A primer designed for metal is still highly recommended if a part is to be exposed to moisture. Once water seeps through to the bare metal, oxidation will begin (plain steel will simply rust). Metal primers may contain additional materials to protect against corrosion, such as sacrificial zinc.
Metal hydroxides/oxides do not provide a solid surface for the paint to adhere to, and paint will come off in large flakes. Using a primer will provide extra insurance against such a scenario. An additional reason for using a primer on metal could be the poor condition of the surface. A steel part can be rusty, for example. Of course, the best solution is to thoroughly clean the metal, but when this is not a viable option, special kinds of primers can be used, which chemically convert rust to the solid metal salts. And even though such surface is still lacking in comparison to the shiny clean metal, it is yet much better than weak, porous rust.
Painting and gluing aluminum is especially important in the aircraft industry, which uses toxic zinc chromate primers and chromating to add the necessary adhesion properties.
[edit] Primers for plastic
Using a primer on surfaces made of plastic is really only necessary when making a drastic change of color (going from dark brown to white, for example), as most household plastics are not very porous and are not easily damaged by moisture, or when a long lasting coat of paint is desired. Basically, a primer will reduce the number of layers of paint necessary to completely cover the previous color, and a primer will help the paint make a thorough bond with the surface being painted. As most paints and primers designed to be used for painting plastics are not water based, an important point for choosing a primer for plastic is making sure primer's propellant or solvent will not dissolve or warp the plastic part itself. Be sure to test both primer and paint on a small hidden spot of the part you are painting.
