· Michael Blair
Primer does jobs that paint cannot. It seals porous surfaces so the topcoat dries evenly, it bonds to slick surfaces that paint would peel from, and it blocks stains and tannins that would bleed through. The phrase paint and primer in one describes a thicker paint with better hiding, which is useful, but it is not a primer and it will not do those three jobs.
Surfaces that always need it
New drywall and fresh joint compound. Bare compound drinks paint at a different rate from the paper face, and without a drywall primer the seams and patches flash through every topcoat, visible in raking light no matter how many coats go on. A PVA drywall primer is cheap and it is the only fix.
Bare wood, especially where it will be painted a light color. Cedar, redwood and many hardwoods carry tannins that bleed through latex paint as brown or pink stains. A stain-blocking primer, oil or shellac based, locks them in. Water-based primers labeled for tannin blocking exist but on cedar and redwood they are the less reliable choice.
- New drywall and any patch or skim coat
- Bare wood, and knots in particular
- Glossy or previously oil-painted surfaces
- Water stains, smoke damage, marker and crayon
- Any drastic color change, particularly to a light color from a dark one
Glossy and oil-painted surfaces
Latex paint does not bond to a glossy oil finish. It looks fine for a few weeks and then peels in sheets when a fingernail catches it. Trim in older Nashville houses is very often oil enamel, and the test is a rag with denatured alcohol: latex softens and comes off, oil does not.
The correct sequence is to scuff sand, clean, and apply a bonding primer rated for glossy surfaces. Then a latex enamel will hold. Skipping the primer and relying on sanding alone is how trim ends up needing to be stripped back to bare wood a year later.
Stains, and why they come back
A water stain on a ceiling, once the leak is fixed and the drywall is dry, will bleed through ordinary paint indefinitely. So will smoke, nicotine, marker, and the tannin ring from a leak on wood. These need a stain-blocking primer, and for the worst of them a shellac-based one, which seals almost anything and dries quickly enough to recoat the same day.
The mistake is painting over the stain with two or three coats of topcoat and hoping. Each coat re-wets the stain and draws it through again. One coat of the right primer and one of topcoat outlasts four coats of paint.
When you genuinely can skip it
Repainting a sound, clean, previously latex-painted wall in a similar color needs no primer. Wash it, fill and spot-prime any patches, and apply the topcoat. This is the situation the paint-and-primer products are actually good at, and where a quality paint with high hiding saves a coat.
Exterior repaints on sound existing paint are the same, provided the surface is clean and chalk-free and any bare spots are spot-primed. In Middle Tennessee humidity, bare wood left unprimed on an exterior for even a few weeks starts to gray and lose its ability to hold paint, so spot priming on the day it is exposed is not optional.