Paint manufacturers nearly always recommend a primer. Yet it would be hard to find a do-it-yourselfer who wants to prime if it’s not necessary. Bare, previously unpainted drywall, though, is one of those surfaces where the answer is unequivocal: Yes, priming is always needed. Drywall soaks up more than its share of paint. If you do not prepare the surface, you may find yourself applying too many coats of paint before the mudded seams stop showing through. If you prime the surfaces first, though, likely you can complete the project with fewer coats of paint—especially if you have used tinted drywall primer. Priming drywall is fast, inexpensive, and effective. Along with a dedicated drywall primer, there are a number of other easy ways to prepare drywall before painting: flat latex paint, hiding paints, and skim-coating with drywall compound.
Taped and mudded seams and screw holes have been covered with drywall compound (mud) and they tend to absorb paint. Drywall is faced with paper. Because paper is porous, it absorbs paint. Overly sanded drywall paper—often, the areas near the mudded seams—may have a scuffed, fuzzy surface that also tends to absorb paint.
The result is that when you paint directly onto bare, finished wallboard, these different rates of absorption will produce a mottled, streaked look where certain areas show through, a condition called flashing. This unevenness only disappears after multiple layers of expensive paint. Depending on the color and glossiness of the paint, it can sometimes take three or even four coats before the painted surface is uniform.
What Drywall Primer Does
Drywall primer helps you begin with one color, instead of several, by standardizing bare drywall’s variegated color scheme. Primer sets another kind of standard: porosity. With added primer, drywall soaks in the paint at one speed, not several speeds.
Primer Equalizes Color
Primer equalizes colors of drywall mud and paper so that the paint colors laid over it can truly shine without interference. Primer’s color sets a base standard for all of the other colors to work off of. With newly finished drywall, you will have two base colors: the color of the paper (gray, off-white, or green) and the color of the drywall compound or mud (white or off-white). One coat of primer or even an inexpensive neutral-colored paint will go a long way towards covering up these colors. A better quality (thicker) drywall primer, also known as a hiding paint, will cover them up completely.
Primer Equalizes Porosity
Drywall primer soaks into the paper, scuffed paper, and mud—the areas of differing porosity—and creates a uniform surface to which the finish paint can adhere. If you have ever looked at a painted wall from a sharp angle and seen the finished joints show through, this is an effect called joint banding or flashing. Drywall primer will significantly reduce or completely eliminate that effect.
Drywall Primer-Sealer
The most common method of priming drywall is to apply a coat of primer-sealer designed for drywall. Drywall primer-sealers come in both water-based (latex) forms, as well as oil-based (alkyd) forms. In addition, you can choose between standard sealers, which are appropriate for perfectly smooth and well-finished walls, or high-build primer-sealers that can help fill in rough or uneven drywall finishing. The high-build products cost quite a bit more, but they may be worth the expense, especially if you are preparing a drywall surface that is rough. To improve coverage and quality, primer-sealers can be tinted before applying, so that the primer coat is a closer match to the color of the finish paint you have chosen. Paint stores may be able to add pigments to the sealer-primer, sometimes for a small additional fee.
Drywall Primer Cost Savings
Though the main benefit of drywall primers is improved aesthetics, they also help you save some money. Drywall primers will cost $15 to $25 per gallon. When compared to builder-grade paint, using a drywall primer will cost about the same. So, there will be no cost savings by substituting a drywall primer coat for a color coat. But the cost savings is considerable with premium paints that cost $60 to $100 per gallon. Drywall primers cost three to four times less per coat, when compared to premium-paint color coats.
Flat Latex Paint
Using flat latex paint is another inexpensive way to prime drywall before painting. Even pros sometimes opt for cheap latex paint as a primer when the surface has been so well finished that the surface is perfectly smooth without flaws. Some drywall manufacturers even recommend plain flat latex paint as one type of drywall primer. The cost of a gallon of basic flat latex paint is usually considerably less than that of top-quality finish paint. As with the primer-sealer, you can tint the flat white latex paint to more closely match the finish color.
Hiding Paints
Hiding paints take the concept of flat latex paint one step further. This product is still a flat latex paint, but it is slightly thicker and has better color-hiding properties than plain latex paint. Make sure to choose a hiding paint that is compatible with unfinished drywall. Hiding paints may cost up to twice as much as ordinary flat latex paint.
Skim-Coating With Drywall Compound
Skim-coating is the process of using a drywall taping knife to scrape drywall compound on and then immediately off. The remaining compound that your knife cannot scrape off is the skim-coat. Skim coating is the level five drywall finishing step that professional installers perform to achieve perfect wall texture. Skim coating cannot be done in lieu of priming; a prime coat would still be needed over a skim coated wall. Defects in the finish will show much more prominently after priming so they can be fixed before finish painting. The surface will also absorb less finish paint, making it go farther.
