Is it really worth the effort and expense to prime before you paint? Absolutely. A primer preps the painting surface and seals any stains so that the paint adheres well, and you’ll get the smoothest, cleanest finish possible. In situations like these, however, neglecting primer can ruin the outcome of your project.
1. You’re painting latex-based paint over oil-based paint.
You know the saying that oil and water don’t mix? Well, the adage holds true for paint. Latex paint, which is water-based, won’t adhere well to a wall that’s already been covered in oil-based paint, unless you prime first.
2. You’re painting brand new drywall.
Fresh drywall soaks up paint like a sponge, and the mudded joints take paint differently than the bare drywall between the joints. Translation: Without a coat of drywall primer, you’re looking at a splotchy paint job. Applying a primer prior to painting drywall will give your finished wall an even, clean look.
3. You’re painting over unfinished wood.
Bare wood is among the trickiest materials to paint. The natural fibers in the wood absorb a lot of paint, and variations in the wood grain can create an uneven finish. So, for the best results, always prime before you paint unfinished wood.
4. You’re painting over a darker color.
Dark interior paint colors are having a moment, but when the time comes that you’re ready to update your walls with a lighter hue, you’ll want to lay a coat of quality paint primer first. Painting a light color over a dark color typically requires multiple coats of paint to keep the old color from showing through the new shade. You can cut down on your work (and your costs) by priming the wall before you paint.
5. You’re painting over wallpaper.
Yes—in fact, you can paint over wallpaper. But you shouldn’t attempt it without priming first. On the other hand, if you’ve already removed the wallpaper, you need to prime the wall to correct any nicks or flaws sustained in the wallpaper removal process, in order to achieve the smoothest final finish possible.
6. You’re painting over a skim coat.
A skim coat is a thin layer of plaster or drywall compound that’s applied to smooth out the surface of a wall. This material is porous, which means it soaks up a lot of paint. To reduce the amount of paint required to cover the wall evenly, you should always prime a skim coated surface before applying color to the wall.
7. You’re painting masonry.
Masonry and brick are very porous surfaces and soak up lots of paint. On top of that, masonry surfaces have a high pH, which makes it difficult for paint to adhere. For this reason, before painting brick or masonry, it’s a good idea to prime it.
8. You’re painting a surface that’s stained.
If the wall you’re painting has had water damage or is stained in other ways, you need to prime before painting. You may even need to use a stain killer primer to ensure unsightly spots don’t bleed through your paint job. Source