Measuring & coverage
Primer: when you need it and when you can skip it
Primer is either essential or a waste of a day, depending on the surface. Here is how to tell which, which primer to buy, and how much.
What primer is for
Primer is a paint designed to be painted over. It is not a cheap first coat of finish paint; it does jobs that finish paint does badly. Depending on the type, a primer does one or more of four things:
- Seals a porous surface, so the finish coats do not soak in unevenly. New drywall, fresh joint compound and bare wood all need sealing.
- Bonds to a surface that finish paint cannot grip, such as old gloss, glossy laminate or tile.
- Blocks stains that would bleed through finish paint: water marks, smoke, nicotine, marker, and the tannins in knotty pine, cedar and redwood.
- Hides a strong colour, so that two finish coats can cover a big change in colour.
If none of those jobs needs doing, you do not need primer. Sound, clean, previously painted walls going to a similar colour take finish paint directly.
When you need it
| Surface | Primer type | Typical coverage | Why |
|---|---|---|---|
| New, unpainted drywall | PVA drywall primer | 300 sq ft/gal | Paper and joint compound absorb differently and “flash” through |
| Patched or skim-coated walls | Drywall or multi-purpose primer | 350 sq ft/gal | Patches show as dull spots under the finish |
| Dark colour going light | High-hide primer, tinted toward the new colour | 300 sq ft/gal | Stops the old colour ghosting through |
| Glossy, slick surfaces | Bonding primer, after scuff-sanding | 350 sq ft/gal | Paint peels from gloss |
| Water, smoke or tannin stains | Stain-blocking primer, shellac or oil for heavy stains | 300 sq ft/gal | Stains bleed through any number of finish coats |
| Previously painted, sound | Usually none | 400 sq ft/gal if used | Nothing to seal, bond or block |
The coverage figures are the typical ones from primer data sheets (Kilz, Zinsser, Sherwin-Williams) for each surface; the primer calculator uses them by default and lets you type in your can’s figure.
When you can skip it
- Repainting sound paint in a similar colour. Wash the walls, fill the nail holes, spot-prime the fillers, and paint.
- A colour change of one or two steps in depth. Two coats of finish paint will hide it. The coats calculator tells you whether your change is one of the big ones.
- Small patches. Spot-prime them rather than the whole wall. The coats calculator allows about 15% of the wall area for spot priming.
- Latex over latex. No bonding primer is needed between two water-based paints in good condition.
“Paint and primer in one”
Many wall paints are sold as “paint and primer in one”. They are thicker, higher-hide formulations that build a heavier film and can cover a moderate colour change in two coats. They are a good finish paint. What they are not is a sealer for new drywall, a bonding primer for gloss, or a stain blocker. On those surfaces, the data sheets for the same products usually still call for a dedicated primer first. Use the self-priming paint where a primer would have been optional anyway, and a real primer where it is not.
How much primer
Primer is one coat, at the coverage for its surface. The formula is the same as for paint:
A 12 × 12 ft room of new drywall
8 ft walls, one door and one window, PVA drywall primer at 300 sq ft per gallon, one coat, 5% allowance.
- Area: 2 × (12 + 12) × 8 − 20 − 15 = 349 sq ft
- Primer: 349 ÷ 300 × 1.05 = 1.22 gallons, buy 1 gallon + 1 quart
- The same room skim-coated, multi-purpose primer at 350: 1.05 gallons, buy 1 gallon + 1 quart
Both figures are from the primer calculator. The room then takes two finish coats at the normal 350 sq ft per gallon, because the primer has sealed the surface; without it, the first finish coat would soak in and cover less.
Tinted primer for colour changes
For a big jump in colour, the cheapest route is often a primer tinted toward the new colour, under two finish coats, rather than three finish coats. Paint counters tint primer on request, usually free. For deep colours, especially reds and oranges, the tint is often a grey, because grey gives the most even base for a saturated colour to cover.
A navy feature wall
A 14 × 8 ft wall with no openings, white going to navy.
- Area: 112 sq ft
- Navy, three coats + 5%: 1.01 gallons, buy 1 gallon + 1 quart
- With a tinted primer: two quarts of primer, and two finish coats would usually do
The figures are from the accent wall calculator, which keeps the three-coat figure as the safe assumption and shows the primer alongside it. Dark colours and LRV explains which colours need this treatment.
Primer for wood, cabinets and metal
- Bare interior wood takes a wood primer, oil-based or a waterborne primer made for wood; knotty or resinous woods need a shellac-based primer over the knots or the resin bleeds yellow through the paint.
- Kitchen cabinets are the classic bonding-primer job: factory finishes are hard and slick, and a bonding primer after a scuff-sand is what keeps enamel from chipping off the door edges. The cabinet paint calculator includes it, and painting kitchen cabinets covers the process.
- Bare steel and iron need a rust-inhibiting metal primer; galvanised metal needs a primer made for galvanised surfaces.
- Masonry and stucco take a masonry primer or sealer, because fresh masonry is alkaline and can break down ordinary paint.
Priming outside
Exterior surfaces follow the same four jobs. Bare wood siding and trim need an exterior wood primer, and cedar and redwood need a stain-blocking one or their tannins bleed through as brown streaks. Spots where old paint has peeled back to bare wood are spot-primed after scraping and sanding; sound old paint is not. New stucco and masonry need time to cure, often around a month, and then a masonry primer that tolerates the alkalinity. Bare galvanised gutters and railings need a primer made for galvanised metal, or the paint will peel within a season. The exterior house paint calculator covers the finish coats once the primer is on.
Choosing between water-based, oil-based and shellac
Water-based (latex or acrylic) primers are the default for walls: low odour, quick to recoat, and fine for drywall, patches and most colour changes. Oil-based primers block moderate stains and tannin better and grip some glossy surfaces, at the cost of odour and a longer wait before recoating. Shellac-based primers dry in under an hour, seal the worst stains and odours, smoke and knots included, and stick to almost anything; they are thin, smell strongly of alcohol while wet, and are best kept to the stains rather than whole walls. The recoat times for each are in the drying and recoat times table.
Common mistakes
- Skipping primer on new drywall. The finish flashes at the seams and the first coat soaks in.
- Using ordinary primer on a water stain. It bleeds through; use a stain blocker.
- Painting over gloss without sanding or bonding primer. The new paint peels in sheets.
- Priming sound painted walls for no reason. It costs a day and a gallon and adds nothing.
- Using white primer under a deep colour. Tint it toward the colour, or grey.
- Leaving primer uncoated for weeks. Most primers are meant to be top-coated within days; check the data sheet.