How we supply it: Primers are arranged through a partner paint dealer who quotes, supplies and bills them and pays us a referral commission. Tell us the stage you are at, bare plaster or putty done, and the partner sends the matching primer with your putty or paint order.
In one line: A wall primer is a thin first coat, thinned with water or solvent, that seals porous plaster or putty and gives the finish paint an even, well-bonded base.
What primer does that paint cannot
A wall primer is a thin first coat, thinned with water or solvent, that seals porous plaster or putty and gives the finish paint an even, well-bonded base.
Fresh plaster is alkaline and putty is thirsty. Without primer, the emulsion is attacked or soaked up unevenly, and the wall shows patches of different sheen.
The full sequence on a new wall
Many sites skip step 2. The primer after putty, step 4, is the one never to skip.
- Cured, dry plaster: commonly about four weeks for new cement plaster (typical).
- One coat of alkali-resistant primer on the bare plaster.
- Two coats of putty, each sanded.
- Once the putty is fully dry, one coat of wall primer.
- Two coats of emulsion, with the gap printed on the can between coats.
Primer types
| Primer | Goes on | Thinned with | Note |
|---|---|---|---|
| Cement (alkali-resistant) primer | New cement plaster, inside or out | Water, per the can | Protects paint from plaster alkali |
| Interior wall primer | Putty or old painted walls | Water | Evens out absorption before emulsion |
| Exterior primer | Exterior plaster | Water or solvent, per the can | Goes under exterior emulsion |
| Red oxide or metal primer | Steel grills, gates, sections | Thinner | Rust inhibition; not for walls |
| Wood primer | Doors and frames | Thinner | Seals grain before enamel |
Water-based or solvent-based?
Water-thinnable primers have low odour, dry fast and suit almost all masonry. Solvent-based primers penetrate deeper and are used where a paint system specifies them, and on metal and wood.
Mixing systems is the risk: a solvent primer under a water-based emulsion, or the reverse, can lift unless the maker says they are compatible.
Litres per room
Typical estimate: for one coat after the dilution printed on the can: about 8–10 m² (85–110 sq ft) per litre on putty. Bare plaster absorbs more.
- 12 × 12 ft room, walls and ceiling (about 584 sq ft): roughly 5.5–7 litres per coat
- One 20-litre can: about 1,700–2,200 sq ft per coat on putty
- Bare new plaster: plan 20–30% more
Checking the primer coat
- Read the can: interior or exterior, water- or solvent-thinned, and the manufacture date.
- Stir to the bottom; settled pigment should mix back without lumps.
- Thin exactly as printed; over-thinning is the commonest primer fault.
- Brush a test patch: dry, it should look evenly matt, without shiny or chalky spots.
- Wipe the dry patch with a damp cloth; nothing should come off.
Keeping cans usable
- Keep cans closed, upright and in shade; heat thickens primer and forms skins.
- Never leave water-based primer in direct sun on the terrace.
- Write the dilution you used on part-used cans.
- Keep solvent primers away from welding and open flames.
Primer mistakes
- Priming damp plaster; trapped moisture blisters the paint later.
- Over-diluting to stretch a can.
- Interior primer on exterior walls.
- Skipping primer after putty, so the emulsion patches.
- Painting the same day as priming in humid weather.