Why Drywall Needs Primer
New drywall has two different surfaces: the paper facing and the joint compound. These absorb paint at different rates.
Joint compound is more porous and absorbent than the paper. When paint hits joint compound, it soaks in. When paint hits paper, more stays on the surface. The result is dull spots over the mud and shinier spots over the paper.
This difference, called flashing, is visible in the finished paint job. Seams appear as dull stripes. Repairs are obvious patches. The whole wall looks uneven.
Primer seals both surfaces to absorb equally. After priming, the paper and joint compound accept paint the same way. No flashing, no visible seams, uniform finish.
Types of Primer for Drywall
PVA Primer
Polyvinyl acetate primer, specifically formulated for new drywall. It's the standard choice and the most economical.
PVA primer seals the porous surfaces without building up excessively. It dries fast, sands easily if needed, and provides a uniform base for paint.
Available in different quality levels. Better PVA primers hide imperfections better and provide better adhesion. For new drywall where you're going to paint with quality topcoat, mid-grade PVA is fine.
High-Build Primer
Thicker primers that fill minor imperfections while sealing. Useful when your mud work isn't perfectly smooth or when you need extra help hiding texture variations.
These cost more and go on thicker. Not necessary for well-finished drywall but helpful for less-than-perfect work.
Shellac and Stain-Blocking Primers
These aren't specifically for new drywall but they handle special situations. Shellac primer blocks water stains, smoke damage, and marker bleed-through. Useful for repairs or when covering problems.
More expensive than PVA and often oil-based, meaning cleanup and odor issues. Use for specific problems, not for general new drywall priming.
Applying Primer
Priming technique isn't complicated, but a few things matter:
Coverage: Don't thin it too much or apply too thin. The goal is complete coverage that seals everything. One solid coat is usually enough, but if you can see porous areas after drying, apply another.
Cutting in: Use a brush around edges, corners, and near trim. A roller for field areas. Same basic technique as painting.
Roller nap: Use a 3/8 inch nap roller for smooth walls. This applies enough primer to seal without creating excessive texture.
Work in sections: Maintain a wet edge to avoid overlap marks. Not as critical as with some paints, but still good practice.
Dry time: Let primer dry completely before painting. Usually a few hours, but check the can. Humid conditions slow drying.
Why Paint-and-Primer Combos Don't Work
Paint companies market paint-and-primer-in-one products heavily. They're fine for repainting already-sealed surfaces. They don't replace dedicated primer on new drywall.
The reason is physics. Joint compound is porous. Paint-and-primer has to be thin enough to level and form a good paint film. That thin consistency lets it soak into joint compound unevenly, just like regular paint would.
True PVA primer is formulated specifically to seal porous surfaces with consistent absorption. Then paint goes over that sealed surface.
I've seen people argue about this. The can says primer, so it should work, right? Try it on new drywall and look at the seams in raking light. Then try proper primer and see the difference. That's the end of the argument.
Priming Repairs and Patches
Repairs on already-painted walls need primer too. The new joint compound is still porous, even if the surrounding wall is already sealed.
For small repairs, I use spray primer. A quick coat over the patched area, dried before touch-up painting. Much faster than pulling out a brush and cleaning it for a small spot.
For larger repairs, brush or roll primer over the new compound, overlapping slightly onto the surrounding painted surface. This ensures a seamless transition.
Shellac primer is useful for repairs involving stains. Water damage, smoke, marker, crayon. Regular primer can't block stains. Shellac encapsulates them.
Common Priming Mistakes
What goes wrong:
Skipping primer entirely: Always results in flashing. The seams show, the patches show, the job looks amateur.
Applying too thin: Primer that's streaked or barely covering doesn't seal. Apply a real coat with full coverage.
Painting before primer dries: Wet primer mixes with paint and can cause adhesion problems. Let it dry completely.
Using the wrong primer for the situation: PVA on stains won't block them. Shellac on new drywall is expensive overkill. Match the primer to the job.
Assuming one coat fixes everything: Usually one coat is enough, but check. Porous areas that still look different need another coat.