Applying a Primer Coat Before Painting
A coat of primer is recommended for almost all painting projects, whether it's new drywall, old wood, bare metal, previously painted brick, or any other surface. Primer is essentially sticky, flat paint that is designed to adhere well and to provide a consistent base for topcoats of paint. If you paint a surface without priming it first, you will likely need more coats for adequate coverage, and the paint may not stick as well to the original surface as it would to the primer. There are different formulations of primer intended for different surfaces.
Benefits of a Primer Coat
Applying primer over new surfaces seals the original material so that the paint doesn't soak into it, requiring extra coats. Primer also helps to hide joints, or seams, on new drywall, and it prevents bleed-through from knots and other natural blemishes and coloring in bare wood. Primer with stain-blocking properties seals over mold stains and other discoloration to prevent them from showing through the finish coats of paint. Primer applied to masonry, metal, and many wood surfaces is essential for proper bonding of the paint job.
Primer is usually white but can be other neutral colors. This is to provide a neutral surface to ensure that paint colors show true. There is no need to color the primer itself, but some paint stores will add a small amount of pigment to primer to make it closer to your final paint color. This is a good idea when the final color is much lighter than the original color of the surface.
An oil-based primer is often recommended for surfaces that are likely to be touched, such as doors, windows, and cabinets. Oil-based primers require mineral spirits for thinning and cleanup. They are excellent for sealing over problematic woods, such as cedar. Shellac-based primers a designed to cover the most challenging surfaces, including smoke stains, crayon, and oil-based adhesives.
Water-based, or "latex," primers are great for blocking stains and even better when the surface has areas that were filled with paste. They provide a great flexible finish with excellent cracking resistance and are recommended to be used on new drywall and bare wood. Before applying water-based primer to bare wood, test it in an inconspicuous area to make sure it does not raise the wood grain. Many water-based primers can also be used on plaster, masonry, brick, and painted metal, depending on the specific formula.
In general, better-quality water-based primers use 100-percent acrylic resins and cost a little more than standard-quality formulas.
A paint-and-primer-in-one product is designed to seal and cover surfaces in one coat. These products work best on new drywall or previously painted surfaces and can provide decent coverage in one coat. However, they are formulated more as thick paint than as primers. This means that they may not perform as well as true primer in many situations. On surfaces that call for the high-performance bond, stain-blocking, or sealing properties of true primer, a paint-and-primer product is not recommended.