Do I really need to use primer before painting?

Sometimes, and skipping it when it is genuinely needed is one of the most common reasons a paint job fails within a year. The useful question is not whether primer is always required but what condition the surface is in.

Primer does three jobs. It bonds to a difficult surface and gives the topcoat something to grip. It seals porous or absorbent material so the topcoat does not sink in unevenly. And it blocks stains or colours that would otherwise bleed through.

You genuinely need it on bare or new surfaces — fresh plaster, bare wood, bare metal, new plasterboard — all of which are absorbent or non-adhesive. You need it when going over a glossy or oil-based finish, where new paint has nothing to key into. You need a stain-blocking primer over water marks, smoke damage, nicotine, crayon or knots in timber, because ordinary paint will not stop them ghosting through. And you need it when covering a very dark colour with a much lighter one, where a tinted primer saves several topcoats.

You can usually skip it when repainting a sound, clean, matt emulsion wall in a similar colour. Washing the surface, filling and sanding matters far more than priming in that case.

Fresh plaster is a special case worth noting. It needs a mist coat — emulsion thinned with around 10–20% water — rather than a conventional primer, so the first layer can soak in and bond. Applying neat paint straight onto new plaster commonly leads to it peeling off in sheets later.

Self-priming paints are real but work best on already-sound surfaces, not as a substitute for a dedicated stain blocker.