A mist coat is simply a diluted coat of emulsion — usually a contract or standard matt thinned with water — painted onto fresh plaster as a primer. New plaster is porous and sucks moisture out of paint like a sponge. Skip the mist coat and your topcoat dries too fast, sits on the surface instead of bonding, and you'll get flaking, peeling, and that dreaded "plastic film" that lifts off in sheets months later. The mist coat soaks in, seals the surface, and gives your proper coats something to grip.
Here's the deal. Wait until your plaster is fully dry — that's a uniform pale pink/buff colour all over with no dark patches. On a skim that's typically 1–4 weeks depending on conditions. Don't rush it.
Then mix roughly 70% paint to 30% water (some go 60/40 on very thirsty plaster). Use a cheap, breathable matt for this — Dulux Trade Supermatt or a basic contract matt is ideal. Do not use vinyl silk, kitchen/bathroom paints, or anything self-priming for the mist coat — they don't breathe and defeat the point.
Apply one or two thinned coats, let each dry, then crack on with your topcoats. Earthborn Claypaint is genuinely brilliant on new plaster because it's highly breathable and very forgiving — worth a look if you want a chalky, eco finish. For a standard matt finish, Little Greene Intelligent Matt or Dulux topcoats sit beautifully over a properly misted wall.
The big "but what about" — can I just buy a dedicated primer instead? Yes. Products designed for new plaster exist and save the faff of mixing, but a watered-down matt is cheaper and works perfectly well. Either way, you're sealing the surface.
One last thing: lay dust sheets. A mist coat is runny and flicks everywhere. Mask the skirting, wear old clothes, and don't fret about coverage being patchy — it's a primer, not the finish. Get it sealed properly and your topcoats will go on like a dream.