Primer (paint)

A primer (/ˈpraɪmər/) 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.

A primer consists of synthetic resin, solvent and additive agent while some primers contain polyethylene (plastic), for better durability.

Primer is a paint product that allows finishing paint to adhere much better than if it were used alone. It is designed to adhere to surfaces and to form a binding layer that is better prepared to receive the paint. Compared to paint, a primer is not intended to be used as the outermost durable finish and 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 aluminum primer, but more often this is achieved through controlling the primer's physical properties such as its porosity, tackiness, and hygroscopy.

In practice, primer is often used when painting porous materials, including concrete and wood. Using a primer is considered 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 from moisture and can prevent the growth of mold. Primers can also be used on dirty surfaces which cannot be cleaned or before painting light colors over a dark finish.

Some primers can be tinted to match more closely 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. Primers are also used to hide joints and seams to give a finished look.

Other primers require that the topcoat be applied within a certain amount of time after the primer dries to ensure optimal adhesion. This varies from 24 hours to up to two weeks after the primer has dried. Painting after the suggested time-frame may cause performance issues depending on the specific situation. It is common to apply the finishing coat of paint before the primer fully cures in order to improve adhesion between the primer and the topcoat. The level of exposure, such as indoors versus outdoors, may affect how important applying the topcoat within the time-frame will be.

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.