Wood is very porous and will absorb the solvent in paint and cause it to dry more quickly. This is undesirable because most paints undergo chemical reactions during the process of curing (for example, latex- and alkyd-based paints polymerise when curing) which is dependent on the water or solvent evaporating slowly. A layer of primer will prevent the underlying wood from prematurely absorbing the solvents in the finishing paint. Primer reduces the number of paint coats needed for good coverage and even color. A thin layer of paint may still be permeable to water. Water can permeate into the wood and cause warping, mildew, or dry rot. Primer improves the waterproofing of the finish. Primers are not used as part of a wood stain treatment because they obscure the wood grain. Primer differs from wood sealers in that sealers typically don't obscure the wood grain completely. Primers are often comparable in price to finish paints and the price is influenced by the quality of the ingredients. Primers for some specialty applications can be expensive. On metal Some metals, such as untreated aluminum, require a primer; 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 might contain additional materials to protect against corrosion, such as sacrificial zinc. Primers are used on metal hydroxides/oxides to improve the surface condition and to encourage bonding. Without a primer, paint applied to metals are prone to peeling. Special kinds of primer (rust-converters) can be used to chemically convert rust to the solid metal salts. This process is not recommended for structural steel. 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. On plastic Using a primer on surfaces made of plastic is only necessary when making a drastic change of color (going from dark brown to white, for example) or when a long-lasting coat of paint is desired. This is because most household plastics are not very porous and are not easily damaged by moisture. A primer will reduce the number of layers of paint necessary to completely cover the previous color, and will help the paint make a thorough bond with the surface being painted. Most paints and