Right, let's clear this up, because there's a lot of confusion here. Bare plaster doesn't need a proper primer in the way bare wood or metal does. What it needs is a mist coat — a watered-down coat of emulsion that soaks into the porous surface and seals it. Get that wrong and your lovely topcoat sits on the surface, can't grip, and either peels off in sheets or flashes (patchy sheen) where the plaster's sucked the moisture out unevenly.
First rule: let it dry fully. New plaster needs weeks, not days — typically 4 to 6 weeks until it's gone from dark patchy brown to a uniform pale pink. Paint it damp and you'll trap moisture and get blooming.
For the mist coat, thin a standard matt emulsion roughly 70% paint to 30% water. Don't use a vinyl-heavy or one-coat paint that won't penetrate — you want it thirsty so it gets pulled in. A trade matt like Dulux Trade Vinyl Matt or Crown Trade Clean Extreme works a treat thinned down. Two mist coats on very porous plaster isn't overkill.
Then build up with your finish coats. If you're going for a premium chalky look, Farrow & Ball Estate Emulsion or Little Greene Intelligent Matt both sit beautifully over a properly sealed wall. Earthborn Claypaint is a cracking choice over fresh plaster too — it's breathable and very forgiving on new surfaces.
"But can't I just buy a tin labelled primer-sealer?" You can, and for tricky jobs — say plaster that's been over-polished and gone shiny, or patch repairs next to old paint — a dedicated sealer like Zinsser Gardz earns its place. But for a normal newly plastered wall, a mist coat does the job and saves you money.
Practical tip: keep a wet edge, work in sections, and don't panic if the mist coat looks rubbish — it always does. It's a foundation, not a finish.