Do I need to prime, and with what?
The four situations that genuinely need a primer, and the ones where it is a wasted coat.
- Difficulty
- beginner
- Time
- 15 min
- Read
- 2 min
Short answer
Prime bare or absorbent surfaces, anything glossy, anything stained, and when going from dark to light. Repainting a sound matt wall in a similar colour does not need it.
Primer is sold as universally necessary and it is not. It does three specific jobs — sealing absorbency, providing grip, and blocking stains — and if none of those apply, it is a coat of paint doing nothing.
Step by step
- Prime bare and absorbent surfaces.New plaster, filler, bare plasterboard, bare wood, bare masonry. These drink paint and dry patchy without something to seal them first.
- Use a mist coat on new plaster.Emulsion thinned with 10-20% water is the traditional and effective approach. Do not use a vinyl silk or a specialist sealer on fresh plaster unless it says it is suitable — plaster needs to breathe as it cures.
- Prime glossy surfaces for adhesion.Existing gloss, varnish, melamine, tiles, laminate. New paint will not grip a shiny surface. Either sand it thoroughly to a key, or use an adhesion primer.
- Use a stain-blocking primer for stains.Water marks, smoke damage, knots in wood, felt tip. These bleed through ordinary paint indefinitely — three topcoats will not cover a water stain, and one stain blocker will.
- Use a tinted primer when going dark to light.A grey or tinted undercoat covers a dark colour in far fewer coats than white primer. Ask the paint counter to tint it towards your topcoat.
- Prime knots in bare wood specifically.Knotting solution before primer. Resin bleeds through paint for years otherwise, and it is a small, cheap, easily-skipped step.
- Skip it where it does nothing.A sound, matt, previously-painted wall being repainted in a similar colour does not need primer. Two coats of topcoat is the right answer.
- Match primer to the surface, not the topcoat.Primers are specific — plaster, wood, metal, tile, stain-blocking. "All purpose" is usually a compromise at everything.
Tips
- Stain blockers are the one product people consistently under-use. A water stain on a ceiling will come through six coats of emulsion and disappear under one coat of blocker.
- Test a small area if you are unsure whether a surface needs priming. Paint it, let it dry fully, and see whether it looks even.
- Solvent-based stain blockers work better than water-based ones on the worst stains. Ventilate well.
Questions people ask
Do I always need to prime before painting?
No. Prime bare or absorbent surfaces, glossy surfaces, stains, and when covering dark with light. A sound matt wall in a similar colour needs no primer.
How do I cover a water stain?
A stain-blocking primer. Ordinary emulsion will not cover it however many coats you apply — the stain bleeds through.