When you need primer (and how much)

The short answer

A clean, sound wall that is already painted usually does not need primer, especially when you repaint with a good interior latex paint in a similar color. Use primer on new drywall, over large patches of joint compound, over water, smoke or tannin stains, on glossy or slick surfaces, on bare wood and for a big jump from dark to light. Primer is estimated like paint: area ÷ coverage per gallon, normally for one coat. Use the paint calculator for the finish coats, then add primer for the surfaces that need it.

Primer labels typically list about 300 to 450 sq ft per gallon. On thirsty surfaces such as new drywall, plan at the low end of the range, because porosity is exactly what lowers the figure.

Surfaces that need primer

New drywall. Benjamin Moore's guidance is that new drywall should always be primed because it is very porous. The paper face and the taped joints absorb paint at different rates, so finish paint rolled straight onto bare drywall tends to dry blotchy, with the seams showing through as dull or shiny stripes. A drywall primer seals both surfaces so the finish coats look even. Bare plaster behaves the same way.

Patched walls. Joint compound and spackle are more absorbent than the painted wall around them. A small nail-hole fill rarely matters, but a large repair can show through the finish as a patch with a different sheen. Spot-prime the repaired areas before painting the whole wall.

Stains. Water marks, smoke, nicotine, marker and the tannins that bleed from knots and cedar can come back through ordinary paint, even after two coats. A stain-blocking primer seals them in. You do not have to prime the whole wall: Benjamin Moore suggests spot-priming just the stains. Heavy water or smoke stains are a job for an alkyd (oil-based) or shellac-based stain blocker, and the cause of a water stain should be fixed before you paint.

Glossy and slick surfaces. Latex paint struggles to grip high-gloss enamel, old oil-based trim, laminate, tile and other non-porous surfaces. Clean and scuff-sand them, then use a bonding primer. If you are not sure whether old trim paint is oil-based, rub it with a cotton ball dampened with rubbing alcohol: latex paint softens and comes off, oil-based paint does not.

Dark to light, or light to dark. A white or tinted primer hides a strong existing color so the new color does not need extra finish coats to cover it. Ask the store to tint the primer toward your finish color when you go from light to deep.

Bare wood. New trim, doors and exterior siding need primer to seal the grain and give the finish an even base. Alkyd primers penetrate bare wood well and block tannin bleed from woods such as cedar and redwood. Outdoors, the guide to painting a house exterior covers priming siding.

Paint-and-primer-in-one products

Many premium paints are sold as self-priming or paint and primer in one. Benjamin Moore explains that these labels are industry shorthand for paints that hide and adhere well in fewer coats over properly prepared surfaces. In other words, they can save a separate primer coat on a sound, previously painted wall, and Benjamin Moore says its premium interior paints can handle most color changes in two coats with no primer.

They are not a substitute for a dedicated primer on every surface. The same manufacturer still recommends a primer for bare surfaces, for stains, for dramatic color changes and for hard-to-coat surfaces such as high gloss, plastic and laminate. A paint-and-primer product rolled onto new drywall is still a coat of finish paint absorbing into a porous surface. The first coat simply acts as an expensive primer, and you may need a third coat to even out the sheen.

If you skip the separate primer and rely on the finish paint, count an extra coat in your estimate. The guide on how many coats of paint you need explains when to plan for two or three.

How much primer to buy

Measure the surfaces that need primer the same way you measure for paint: wall area is 2 × (length + width) × height, minus about 20 sq ft per door and 12 sq ft per window, and a ceiling is length × width. For patches and stains, add up only the areas you will spot-prime, then round up generously, since a quart is the smallest practical purchase.

One coat of primer is the norm. Divide the area by the label coverage, and use the low end of the range on porous surfaces. Coverage varies by product. KILZ lists its PVA drywall primer at 300 to 400 sq ft per gallon depending on porosity, Benjamin Moore lists its Ultra Spec 500 interior primer at 350 to 400 sq ft per gallon, and its Fresh Start High-Hiding All Purpose Primer at 400 to 450. That is similar to or a little below the 350 to 450 sq ft per gallon typical of finish paints, so do not assume a gallon of primer stretches further than a gallon of paint.

Primer does not replace the finish coats. Plan the finish paint as usual, normally two coats. See how paint coverage works for why surfaces change the coverage figure.

Worked example

A newly finished basement room is 14 ft long and 12 ft wide with 8 ft walls, one door and two windows, all bare drywall. The perimeter is 2 × (14 + 12) = 52 ft, and the gross wall area is 52 × 8 = 416 sq ft. The openings take off 20 + 2 × 12 = 44 sq ft, leaving 416 − 44 = 372 sq ft of wall.

For one coat of drywall primer, plan at the low end of a 300 to 400 sq ft per gallon label: 372 ÷ 300 = 1.24 gallons, which rounds to 1.2. That is one gallon and one quart. The ceiling is 14 × 12 = 168 sq ft; if it is bare drywall too, the total to prime is 372 + 168 = 540 sq ft, and 540 ÷ 300 = 1.8 gallons, which is two gallons.

The finish coats for the walls are separate. Two coats at 400 sq ft per gallon need 372 × 2 ÷ 400 = 1.86 gallons, which rounds to 1.9: two gallons. If you skip primer and choose the new drywall setting in the paint calculator, it reduces coverage to 320 sq ft per gallon (400 × 0.8), and two coats need 744 ÷ 320 = 2.325 gallons, which rounds to 2.3: two gallons and two quarts, and unprimed drywall may still need a third coat to even out the sheen.

Now a different job: a previously painted dark red bedroom wall of 372 sq ft going to off-white. One coat of high-hiding primer at the low end of 400 to 450 sq ft per gallon needs 372 ÷ 400 = 0.93 gallon, which rounds to 0.9: one gallon, and the two finish coats then go on over an even, light base.

Sources

• Benjamin Moore, FAQ: Do I need to prime a wall prior to painting it? and Does Regal paint have primer in it?
• Benjamin Moore, How to Choose the Best Paint Primer
• Benjamin Moore, Fresh Start High-Hiding All Purpose Primer product page (400 to 450 sq ft per gallon)
• Benjamin Moore, Ultra Spec 500 Interior Primer product page (350 to 400 sq ft per gallon; new drywall and plaster)
• KILZ, PVA Drywall Primer (300 to 400 sq ft per gallon depending on porosity)