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A primer (//) or undercoat 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.
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 form a durable finished surface, they can instead be engineered to have improved filling and binding properties with the material underneath. Sometimes this is achieved by chemistry, as in the case of aluminium primer, but more often this is achieved through controlling the primer's physical properties, such as porosity, tackiness, and hygroscopy.
In practice primer is often used when painting many kinds of porous materials, such as concrete and especially wood (see detailed description below). Priming is mandatory if the material is not water resistant and will be exposed to the elements. Priming gypsum board (drywall) is also standard practice with new construction because it seals the wall and aids in preventing mold. Primers can also be used for dirty surfaces which cannot be cleaned, or before painting light colours over existing dark colours.
Primers can usually be tinted to a close match with the colour of the finishing paint. If the finishing paint is a deep colour, tinting the primer can reduce the number of layers of finishing paint that are necessary for good uniformity across the painted surface. Primers are also used to hide joints an seams to give a finishing look.
There may be a maximum time-frame within which a topcoat should be applied over the primer after the primer dries, in order to achieve maximum performance. Depending on the primer, the next coat of paint should be applied as quickly as 24 hours or as long as two weeks. Painting after the suggested time-frame may cause performance issues depending on the specific situation. Painters often apply the finish coat of paint before the primer fully cures in order to increase adhesion of the topcoat to the primer. If top coating is applied after the suggested time frame, consider using a "self priming" topcoat. For definitive answers on recommended repainting time-frame, check the primer label/website, or contact the manufacturer directly. Recoat time-frame is most likely a more critical factor in exterior application because of the more extreme climatic exposure.
Using a primer on wood before painting is important for several reasons. First, wood is very porous and will absorb the solvent from paint, drying the paint prematurely. Because most paints undergo chemical reactions during the process of curing (for example, latex and alkyd-based paints actually polymerise when curing), they depend on water or solvent being evaporated slowly rather than being absorbed quickly by the underlying material. A layer of primer will help the paint to undergo its proper, complete curing cycle.
Second, without a primer, several layers of paint can be necessary to completely obscure the wood grain and ensure even colour.
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. Primer adds to the waterproofing effect of the paint.
Quality primers are often comparable in price to finish paints, their cost influenced by the quality of binders that they use. Some specialty primers are in fact quite costly.
Primers are not used for a wood stain treatment that is designed to show the wood grain. On soft woods, a wood conditioner (thinned shellac or varnish) allows for more even colouring of stain. Sealers are designed to promote uniform finishes. They are designed with qualities that promote quick drying and they have high isocynate content and are not sandable,
Some metals, such as untreated aluminium, 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.
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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 (blasting), but when this is not a viable option, special kinds of primers can be used that 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.
Using a primer on surfaces made of plastic is only necessary when making a drastic change of colour (going from dark brown to white, for example), because most household plastics are not very porous and are not easily damaged by moisture; or when a long-lasting coat of paint is desired. A primer will reduce the number of layers of paint necessary to completely cover the previous colour, and will help the paint make a thorough bond with the surface being painted. Because 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 the primer's propellant or solvent will not dissolve or warp the plastic part itself (e.g. most common household spray paint will damage polystyrene foam). Both the primer and paint should be tested on a small hidden spot of the part being painted.
Alternative surface treatments, such as plasma activation, can replace primers in the cases when the latter are used to improve adhesive bonding between the substrate and the paint or the lacquer. Quality of the adhesive bonding, such as varnishing and painting, depends strongly on the ability of the adhesive to efficiency cover (wet) the substrate area. This happens when the surface energy of the substrate is greater than the surface energy of the adhesive. However, high strength adhesives – lacquers and paints – have high surface energy. Thus, their application is problematic for low surface energy materials such as polymers or oxidized metals.
To solve this problem, plasma activation is used as a preparation step before adhesive bonding. It cleans the polymer surface from the organic contaminants, removes a weak boundary layer, strengthens the surface by cross-linking polymer molecules and chemically bonds to the substrate a strong layer with high surface energy and chemical affinity to the adhesive. Moreover, plasma processing can also reduce or remove hard oxides from metal surfaces, enabling painting and gluing of metals such as copper and aluminium. Importantly, plasma activation can be performed at the atmospheric pressure in air with fast processing speeds. It does not use wet chemistry, which positively affects its costs, safety and environmental impact.
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