Primer is not a rule, it is an answer to a problem
Ask around and you will be told that you always prime. That advice is safe, it sells more paint, and on a large number of jobs it is simply wrong. A wall that has been painted before, is sound, is clean and is being repainted a similar colour does not need a primer coat. Putting one on costs you a day and a tin and buys nothing.
The paint calculator on this site defaults primer to *not required* for exactly that reason. It is a question you answer, not a line that appears on every estimate whether it applies or not.
What primer actually does is solve one of four specific problems. If none of them are present on your wall, you do not have a problem to solve.
The four problems primer solves
- Absorption: bare plaster, new plasterboard, bare timber and masonry drink the first coat unevenly. The result is a patchy finish where the porous areas look flat and dead and the sealed ones look normal. A primer or a thinned mist coat evens out the porosity so the topcoats sit on a consistent surface.
- Adhesion: some surfaces give paint nothing to grip. Gloss paintwork, melamine, tile, metal and PVC all fall into this category. A bonding primer sticks to the difficult surface and gives the topcoat something it can hold onto. Skipping it here is how paint ends up peeling off in sheets a year later.
- Stain blocking: water marks, nicotine, smoke damage, marker pen and the tannins in new timber will bleed through emulsion no matter how many coats you apply. They need a stain-blocking primer, which is usually shellac or solvent based, because the point is to seal the stain behind a barrier the water-based topcoat cannot dissolve.
- Colour change: going from a dark colour to a pale one, or covering a strong colour like a deep red, can take four or five coats of finish paint. A grey or tinted primer gets you to a uniform base in one, and finish paint is more expensive per litre than primer.
When you can genuinely skip it
- A repaint over sound, matt existing paintwork in a similar colour. Wash it, fill and sand what needs it, and go straight to the finish coat.
- A second coat of the same paint you applied yesterday.
- Any manufacturer system where the finish paint is explicitly sold as self-priming for that substrate. Read the data sheet rather than assuming, but these do exist and they work.
The common thread is that the surface is already sealed, already stable, and already grips paint. There is nothing left for a primer to do.
The mist coat on new plaster
New plaster is the case people get wrong most often, and it is worth being precise about. Fresh plaster is very absorbent and highly alkaline, and a full-strength coat of emulsion applied straight onto it can dry too quickly, fail to key, and peel away in sheets when you take the tape off.
The traditional answer is a mist coat, which is ordinary matt emulsion thinned with water — commonly around three parts paint to one part water, though the tin will tell you what the manufacturer wants. Thinned paint soaks in rather than sitting on top, sealing the surface so the following coats behave normally.
Two things to watch. Let the plaster dry properly first. It should be uniformly pale with no dark patches, and depending on thickness and conditions that can take several weeks, not several days. And do not use a vinyl silk or a paint containing PVA for the mist coat, as those form a film rather than penetrating, which is the opposite of what the job needs.
How much to order
Primer is quoted as a single coat. The calculator works it out at the same texture-adjusted rate as the finish paint — one coat over the same net area you measured, plus the same waste allowance — so on a room needing 3 litres per finish coat, the primer is about 3 litres too.
Two adjustments worth making yourself. On bare, thirsty stock the first coat goes further into the surface than a coverage table suggests, so round generously rather than tightly. And if you are priming for colour change rather than for absorption, ask the merchant to tint the primer towards your finish colour. It costs almost nothing and can save you a full coat of the expensive tin.
The honest summary is this. Primer is not a virtue and it is not a formality. It is a specific fix for absorption, adhesion, stains or colour. Look at your wall, decide which of those you actually have, and price accordingly.