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.

20%-30% synthetic resin, 60%-80% solvent and 2%-5% additive agent. Some primer contains polyethylene (plastic), for better durability.

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 improved filling and binding properties with the material underneath. Sometimes, this is achieved with specific 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 that, for some reason, cannot be cleaned, or before painting light colors over existing dark colors.

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.

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 timeframe may cause performance issues depending on the specific situation. Supposedly, you want to apply the finish coat of paint before the primer fully cures on a molecular level. Doing this allows maximum adhesion/bonding of the topcoat to the primer. If topcoating after the suggested timeframe, consider using a "self priming" topcoat. For definitive answers on recommended repainting timeframe, check the primer label/website, or contact the manufacturer directly. Recoat timeframe is most likely a more critical factor in exterior application because of the more extreme climatic exposure.

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Wikipedia