How many coats of paint you really need

Why coats matter more than coverage

Paint quantity scales directly with coats. Going from two coats to three adds half again to the paint; going from two to one halves it. No other input in a paint estimate moves the answer that much, and it is the one people guess. The rules below are the ones manufacturers publish in their application guidance, and they come down to two jobs a coat of paint has to do: hide what is underneath, and build a film thick and even enough to look uniform and last.

The rules

The coats calculator applies this table to your old colour, new colour and surface, and turns the result into paint for your area.

One coat: only for a refresh

One coat works when the new paint is the same colour as the old and the old paint is sound. It freshens a scuffed wall without changing anything. It does not work for a colour change, however close the colours look on the chip, because one coat of a new colour almost always shows the old one faintly through it, especially at the edges where you cut in with a brush.

“One-coat” paints exist and are thick, high-hide formulations. They can cover a similar colour in one coat on a smooth wall. They are not a reliable way to change colour, and their data sheets usually still recommend two coats for the best result.

Two coats: the rule for any colour change

Two coats is the standard because the first coat never goes on perfectly evenly. It is thinner where the roller ran dry and thicker where it was loaded, the cut-in edges dry differently from the rolled field, and the old colour shows through unevenly. The second coat evens out the film thickness, the colour and the sheen. It also builds the film to the thickness the paint was designed for, which is what gives it its washability and durability.

Three coats, or a primer: the big jumps

Some colour changes are too big for two coats to hide. The usual cases:

• Light to dark. A navy or charcoal over a white wall. Dark colours are mixed in deep bases with little white pigment, and white pigment is what hides. Two coats may still look patchy.
• Dark to light. A white over a red or a navy. The old colour ghosts through the new.
• Saturated colours. Reds, oranges and yellows are the worst hiders in the store, even over a light wall.

The choice is a third coat of finish paint or one coat of primer under two coats of finish. A primer tinted toward the new colour, or grey for deep colours, gives the finish coats a closer base to cover. For a dark-to-light change, a high-hide white primer blocks the old colour. Primer usually costs less per gallon than the finish paint it replaces, and it often gives a more even result.

The surface can add a primer too

Some surfaces need primer whatever the colour, because paint will not hide or stick properly without it:

• Bare drywall absorbs unevenly; the joint compound and the paper take paint differently and show through as “flashing”. A PVA drywall primer seals it.
• Patches show as dull spots under the finish unless they are spot-primed. Spot priming is usually about 15% of the wall.
• Stains from water, smoke or knots in wood bleed through any number of coats of ordinary paint. A stain-blocking primer, shellac or oil-based for heavy stains, is the only fix.
• Gloss gives paint nothing to grip. Scuff-sand it and use a bonding primer, or the new paint will peel.

The primer calculator works out the primer for each surface, and primer: when you need it covers each case in detail.

How depth is judged

Colour depth is judged by lightness, not hue. A chip’s light reflectance value (LRV), printed on the back of most US paint chips, runs from 0 for black to 100 for pure white. A change of about two steps on a scale of white, light, medium, dark and very dark is where two coats stop being enough. A medium beige to a dark brown is one step and takes two coats; a pale grey to a charcoal is two steps and needs the primer or the third coat. Dark colours and LRV has the full table.

Accent walls and small jobs

A single dark wall is where the third coat surprises people most, because one gallon looks like plenty. A 14 × 8 ft wall in navy at three coats is just over a gallon, and a gallon alone leaves nothing for the cut-in edges and touch-ups. The accent wall calculator applies the depth rules to one wall and adds the tinted primer and sample pots.

Ceilings, trim and outside

The same rules apply to every surface, with a few local habits:

• Ceilings in the same white often get away with one coat of a high-hide ceiling paint, because nobody looks at a ceiling in raking light from close up. A ceiling going from yellowed to white, or with water stains, needs a stain-blocking primer first.
• Trim and doors in enamel usually take two coats over a sound, sanded old finish. Over bare wood, prime first; over old oil-based gloss, scuff-sand and use a bonding primer.
• Exteriors take two coats on bare or weathered wood and on any colour change. One coat of a good exterior paint over sound paint of the same colour is a common maintenance repaint on a sheltered wall, not on the sunny south and west faces.

Common mistakes

• Planning one coat for a colour change. The old colour shows at the edges and in raking light.
• Expecting “paint and primer in one” to replace a real primer on bare drywall, stains or a big colour jump. It thickens the film; it does not seal or block.
• Using white primer under a dark colour. Tint it toward the new colour, or grey.
• Skipping the second coat because the first looks fine wet. Wet paint looks more even than it dries.
• Recoating too soon. Wait for the recoat time on the can, longer in a cold or damp room; see dry, recoat and cure.

Once you have the coats, the wall paint calculator turns them into cans for the room.