How Many Coats of Paint Do You Need?

The two-coat default

Paint is sold as if one coat were normal. It is not. Take one professional wall paint as an example: Sherwin-Williams’ data sheet for ProMar 200 Zero VOC flat gives 350 to 400 square feet a gallon at 4 mils wet, which dries to 1.4 mils — one thin film. Every system that same sheet specifies, over drywall, block, masonry, plaster or wood, is a primer and two coats.

A one-coat wall looks fine on the day and shows its thinness sooner than it should: burnish marks where a chair touches it, a dull patch where you wiped a mark off, uneven sheen in raking light. None of that is a defect in the paint. It is an under-built film.

The second coat also costs less than the first in labour terms. The cutting-in is faster because the line is already there, the roller loads more evenly on a sealed surface, and you are not stopping to fix holidays. On a normal bedroom the second coat goes on noticeably faster than the first, for the same paint.

So plan two coats and buy for two coats. The paint calculator defaults to two and rounds up to the next half gallon.

When one coat works

Four conditions, and you need all four at once:

1. The wall is already painted, clean and sound.
2. The new colour is within a shade or two of the old one.
3. You are using a premium self-priming paint, not a contractor-grade flat.
4. There are no repairs, patches or stains anywhere on the wall.

That is a repaint of a bedroom from one soft grey to another. Everything else is two.

Ceilings are the honest exception: a flat white ceiling being repainted flat white, with no stains, takes one coat plus spot work, and a flat sheen hides the lap marks a single coat leaves.

One warning about single-coat work: it only holds if the sheen matches too. A one-coat eggshell over an old flat will look patchy wherever the old surface absorbed differently, and that patchiness only shows up once the paint dries, by which point you are committed.

When you need three

Three coats of finish paint is almost always the wrong answer to a problem that primer would have solved. But there are cases where it is unavoidable.

Notice that in five of those six rows, the answer is a primer plus two coats rather than three coats of finish. At this site’s default prices primer is $32 a gallon against $42 for finish paint, and it is made to stick and to seal rather than to look good. The data sheet above says the same of colour: when a repaint involves a drastic colour change, a coat of primer improves the hiding of the topcoat.

Primer vs an extra coat

A primer and a paint do different jobs. Primer seals a porous surface so the finish coat sits on top instead of soaking in, it sticks to difficult substrates, and it blocks stains from bleeding through. A finish coat provides colour, sheen and washability.

Where people go wrong is using "paint and primer in one" as if it were a primer. It is not. It works over a sound painted wall. Over bare drywall, bare wood, a stain or a glossy surface, it behaves like paint, because it is paint.

Match the primer to the problem, not to the brand of the finish coat:

The rule that holds up: prime when the surface changes what the paint does. Bare, porous, stained, glossy or a drastic colour change — prime. Otherwise, two coats of a good paint.

Sheen and touch-ups

Sheen decides how forgiving a wall is. Flat hides surface imperfection and cannot be wiped clean. Eggshell and satin wipe well and show every roller lap. Semi-gloss is for trim and bathrooms.

Sheen also decides whether a touch-up will be invisible. Flat touches up well — you can dab a mark with a brush and it disappears. Satin and semi-gloss almost never touch up: the repaired spot dries at a different sheen and the patch is visible from across the room. On those, plan to repaint the whole wall corner to corner.

Three habits that matter more than the number of coats:

1. Keep a wet edge. Cut in one wall and roll it immediately, rather than cutting in the whole room first. A dry cut line flashes differently from the rolled field.
2. Respect the recoat time. It is on the can: the data sheet above gives 4 hours at 77°F and 50% humidity, and says drying and recoat times change with temperature, humidity and film thickness. Rolling over a half-cured coat pulls it off the wall.
3. Box your paint. Mix every can into one pail before starting. Tint varies between cans and the difference shows on a long wall.

Keep a labelled quart of each colour when you finish, with the room, the sheen and the formula sticker from the can lid. Touch-up paint from the same batch is the only touch-up paint that ever matches.

Once you know the coats, the gallons follow: the arithmetic for a whole house is in how much paint for 1,000 square feet. If the walls are new board, plan the primer with the finishing schedule in how many sheets of drywall — fresh joint compound is the thirstiest surface in the house and it will drink a coat you were counting on for colour.