The question of whether or not to apply paint primer before the color coat is difficult because everyone, it seems, has a different agenda. Paint manufacturers nearly always recommend that you brush, roll, or spray one or two coats of paint primer since this is in service of producing a better color coat. Paint contractors charging by the hour might recommend a primer, while contractors charging by the job might lean toward no primer.
Do-it-yourself painters nearly universally want to skip priming, if at all possible. Often, the answer is less based on objective factors than on subjective ones such as mood, cost, time, and one's love-hate relationship with painting. After all, priming can feel like a wasted effort. Priming is as much work as laying down the color coat. Every stroke of the brush and every roll of the roller is the same as you would do for the eventual color coat.
What Does Paint Primer Do?
Paint primer is designed to provide a stable surface that subsequent paint layers can lock onto. Paint primer also helps to hide surface stains.
A surface's porosity is the condition that most often warrants the use of a paint primer. When the surface is porous, too much of the paint will draw into the surface. The opposite can be a problem, as well. When the surface is too glossy, color coat adhesion is difficult because the paint bodies cannot lock onto anything. Paint primer is also valuable for covering up lower stains. This lets the color coat do its job of providing beautiful colors instead of covering up stains.
You can never go wrong with priming. If you have little confidence about the condition of the wall prior to painting, the default choice is to prime it.
When You May Need Paint Primer
Not all conditions need to be present for you to decide to prime the surface first. Common triggering events include:
Prime When the Surface Is Porous
A highly porous surface usually means that primer is needed. Newly installed drywall is highly porous in two ways: the bare facing paper on drywall and the dried joint compound covering the seams. Bare wood is even more porous and always requires a primer. Masonry such as retaining wall blocks and bricks need paint primer.
Prime When the Drywall Is Skim-Coated
A skim coat is a thin swipe of drywall compound laid over bare drywall. Considered a level five finish, the highest grade possible, a skim coat is not something you encounter often. But as with bare wood or drywall paper, it is highly porous and thus requires a primer.
Prime When the Previous Coat Is Glossy
Glossy base coats do not hold paint well. A light scuffing with sandpaper and a coat or two of primer will help the color coat stick. Even if you decide not to scuff that glossy sheen, using primer will help subsequent coats stick.
Prime When Changing From Dark to Light Colors
Avoid the problems that come with repeatedly laying down expensive light-colored paint over darker colors. Instead, first treat the surface with two layers of white primer if the existing color is extremely dark.
When going from a light color to a dark color, note that most paint retailers have the ability to tint your primer. This brings the color of the primer closer to that of the wall finish color, reducing the number of primer coats and color coats you lay down.
Prime When the Surface Is Stained
Spotted or stained surfaces benefit from a coat or two of priming before painting. Consider using thicker primers such as Kilz 2 or Kilz Max for these conditions.
When You May Not Need Paint Primer
While priming is usually the best bet, you can often get by without priming under any of the following conditions:
Clean Walls Usually Do Not Need Priming
If your walls are perfectly clean and in good condition, you might be able to eliminate the need for primer. One way to clean walls before painting is to create a thin mixture of tri-sodium phosphate (TSP) and water and wipe down the walls with a soft cloth. If you choose not to use TSP, attach a clean brush attachment to the end of a shop vacuum and clean off major debris like cobwebs and dust.
Same-Color Walls May Not Need Priming
One need for primer is to readjust the base color for your new color to brilliantly and accurately show up. If previous and new colors are the same, the need for primer is reduced though not entirely eliminated.
Paint-Primers May Eliminate Priming
Self-priming paint is essentially paint that is thicker than regular paint. Because it is thicker, it builds up higher and forms a thicker coat. It is preferable to use separate primer and paint. But if the walls are basically in good condition, you can use a combination paint and primer. Self-priming paint is not the miracle cure that many homeowners may believe. Laying down a thicker paint build makes for a weaker coat that takes longer to dry.