Primer
Also called: priming, primer coat, paint and primer in one, self-priming paint, stain-blocking primer, bonding primer, spot priming
An undercoat made to bond to the surface and give the finish paint an even, sealed base. Needed on bare drywall, patches, stains, glossy surfaces and dark-to-light color changes; not needed over sound, clean, previously painted walls in a similar color. Paint-and-primer covers only that last case.
Paint is built to look good and wear well; primer is built to stick and to seal. Where the surface under the paint is uneven in porosity or color — bare drywall, a spackle patch, a water stain, a glossy old finish, a red wall going white — the finish paint alone either soaks in unevenly, doesn’t grip, or lets the old color show through. Primer evens the base so two coats of paint look like two coats everywhere.
The types that matter in a house: a drywall primer (PVA) for new board; a stain-blocking primer, shellac- or oil-based, over water stains, smoke, marker and knots; a bonding primer over glossy paint, laminate or tile; and a tinted primer, gray or close to the finish color, under a deep color or over one. Spot priming is priming only the patches and stains rather than the whole wall — always required over spackle, which otherwise drinks the paint and flashes as a dull spot in every light.
“Paint and primer in one” is a thicker paint with better hiding, and it does replace a primer coat over sound, previously painted walls in a similar color. It doesn’t replace primer on bare drywall, patches, stains or gloss, whatever the can implies. The cost page prices a full primer coat as the add-on it is when a painter quotes a color change.