To prime or not to prime, that is the question. The answer is almost always yes if you want a richly colored, uniformly smooth, long-lasting finish.
It may seem to be an unnecessary extra step when tackling a painting project, but priming a surface to be painted can actually save time in the long-run and produce a superior result. What are the benefits of priming before painting a surface? Primers can help create a beautiful, uniform, and long-lasting final coat by covering stains and discoloration, hiding surface marks and darker colors, sealing porous surfaces, filling and leveling uneven surfaces, and enhancing the color and adherence of the finishing coat of paint.
If a surface is discolored by water stains or mildew spots, a primer is needed. If a surface has not been painted previously or has been patched with spackling, a primer is needed. In fact, a primer is recommended for use before painting any type of surface, interior, or exterior, unless the surface is already sealed or painted and is in good condition. It should also be kept in mind that the surface properties of primers can deteriorate over time, and thus a primed surface should be painted within one week for maximum performance.
The question then becomes which primer to use.
Choosing the appropriate primer is critical to achieving a high-quality finish. Several factors should be considered when selecting a primer: the type of paint to be used, the type of surface to be painted, and the specific condition of the surface.
As a general rule, oil-based primers are used with oil-based topcoats, and latex-based primers are used with latex topcoats, although latex primers can also be covered with oil-based paints. Fortunately, the availability of the popular universal oil/alkyd primers which may be used with either oil-based or latex paint has significantly simplified the oil-or-latex decision.
The type of surface to be painted also influences the choice of primer. Primers have been designed to meet the requirements of each different type of surface, whether it is plasterboard, wood, masonry, or metal.
General-purpose primers are useful for sealing most porous surfaces, such as internal walls and ceilings. These primers are widely available in-store aisles next to topcoat paints. They generally require about four hours to dry but should be allowed to cure for about 16 hours before the finish coat is applied.
General-purpose primers are suitable for chipboard, hardboard, plaster, plasterboard, plywood, and softwood. Wood primers are similar to general-purpose primers in that they prevent further coats of paint from soaking into the wood surface. Wood primers can be used on chipboard, hardboard, plywood, and softwood.
For concrete, stone, or brick surfaces, a primer seals the masonry against water penetration. Water repellent primers or sealers designed for masonry dry colorless within one hour, but should be left for 12 hours before being covered with a coat of paint.
Metal surfaces must be protected from corrosion and rust in order to produce a long-lasting painted finish. Primers containing zinc phosphate are suitable for use on galvanized metal, aluminum, ferrous metals, new steel, and new iron.
Zinc phosphate primers dry within approximately four hours and can be painted after about ten hours. An alternative primer for use only on aluminum is a chromate primer, but this primer requires nearly ten hours to dry and about 24 hours to cure sufficiently for the application of the finish coat.
Primers have been designed to meet specific surface challenges. One common surface imperfection is mildew, often accompanied by water stains. To determine if a stain is a mildew or dirt, a small drop of bleach can be applied to the spot. If the discoloration tends to disappear, it is mildew.
Certain primers contain chemicals called mildewcides that help stop mildew growth on surfaces. Before applying a primer, the mildewed surface should be cleaned with a solution containing equal volumes of bleach and water, rinsed well with water, and allowed to dry. It is best to prime and paint mildew-prone surfaces during the dry season.
Some types of wood present problems that are more unique. For example, knotty, resinous wood can be difficult to seal before painting. However, there is a primer designed for use on knotty, resinous wood. The high shellac content of this knotting primer prevents resin from leaking through to the paint finish. Oily hardwoods can be sealed with aluminum wood primers.
The proper use of the appropriate primer will enhance the appearance and endurance of a final painted surface, which, in the end, always saves time and effort.