Priming before painting can be a vexing question, because it is not always necessary, even though paint companies recommend that you always prime. Often, your answer seems less based on objective factors than on subjective ones: mood, tolerance for painting, and cost.
After all, priming can feel like a wasted effort. Priming is as much work as laying down the finish color coat. Every stroke of the brush, every roll of the roller, every linear inch of painter's tape stuck down is the same as you would do for the eventual color coat. If you think you can breeze through this stage, think again.
When You May Need to Prime
Not all conditions need to be present for you to decide to prime the surface first:
1. Surface Is Bare Wood or New Drywall
This is the biggest "yes" of them all. Newly installed drywall is highly porous in two ways: the bare facing paper on drywall and the dried joint compound ("mud") covering the seams. Bare wood is even more porous and always requires primer.
2. Drywall Surface Is Skim-Coated
A skim coat is a thin swipe of drywall compound laid over bare drywall. Considered a level 5 finish (the highest grade possible), it's not something you encounter often. But as with bare wood or drywall paper, it is highly porous and thus requires primer. Wall texture is a material much like drywall skim and also would require priming.
3. Previous Coat Is Glossy
Glossy base coats don't hold paint well. A light sanding and two coats of primer will help the color coat stick. If you decide not to sand down that glossy sheen, using primer will help subsequent coats stick.
4. Changing From Dark to Light Colors
Avoid the heartache that comes with repeatedly laying down expensive light colored paint over darker colors. Instead, first treat it with two layers of white primer; three, if the existing color is extremely dark.
When going the opposite direction--from light to dark--note that most paint retailers have the ability to tint your primer. This brings the color of the primer closer (or even matching) that of the wall finish color, reducing the number of primer coats you lay down.
5. If Uncertain, Prime the Surface
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 Not Need to Prime
While priming is usually the best bet, you can often squeak by without priming under any of the following conditions:
1. Walls Are Clean
Primer tends to stick better to walls in imperfect condition more than will paint. So, if your walls are perfectly clean, it helps to eliminate the need for primer. One way to clean walls before painting is to create a thin mixture of 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.
2. New Coat Matches Previous Coat Color
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, once again the need for primer is reduced, though not entirely eliminated.
3. Priming Prevents Painting
This one is all about psychology. Are you putting off painting because you don't want to prime? If painting without priming is the nudge you need to get to work, then do it. Painted surfaces are always better than raw surfaces, in terms of appearance, durability, and maintenance.
4. You Are Using Paint and Primer Combination
Self-priming paint can be summed up succinctly: it is a 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 more or less 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 thicker paint "build," as it's called within the industry, makes for a weaker coat that takes longer to dry.