How long should plaster dry before painting an extension?

In a warm, ventilated room a skim coat can be ready in 5-7 days, but a newly built extension drying out blockwork and screed at the same time often takes four weeks or more. The reliable test is colour: dry plaster is a uniform pale cream, and any dark patches mean it is still too wet to paint.

What is a mist coat and why is it needed on new plaster?

A mist coat is a watered-down coat of non-vinyl matt emulsion that soaks into fresh plaster and seals the surface so the topcoats can grip. Without it the porous plaster drinks the binder out of full-strength paint, leaving a powdery finish that peels.

Can you use any emulsion for a mist coat?

No. The mist coat must be non-vinyl matt emulsion, because vinyl matt and silk paints form a film on the surface rather than bonding into the plaster and will eventually fail.

Why does a decorator visit several times instead of finishing in one go?

Rooms become ready for paint at different times as plaster dries and other trades clear out, so a decorator paints what is ready, leaves, and returns when the next room is available. Four or five visits across several weeks is normal on a live build, not a sign of poor organisation.

What is the standard for judging decoration snagging defects?

The accepted test is viewing the wall from two metres away in natural daylight. If a run, missed patch, or uneven coverage is visible at that distance it counts as a defect; if it is not, it does not.

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Phase 7 · Second Fix · Task 08 of 09

Decorating Your Extension: Painting and Finishing

Decoration for a typical extension costs £1,500 – £2,500. How the mist coat works, why your decorator visits 4 times instead of once, and what to snag before you sign off.

10 min readUpdated 2026Premium

Process infographic showing the decoration sequence for a new extension: plaster drying check, mist coat application, woodwork priming, walls and ceilings two coats, snagging walk-through, with timing markers for each stage

Prerequisites

Complete these first

Paint over damp plaster and nothing happens for a few months. Then it starts. Bubbling. Hairline cracks. Small sections peeling at the edges. You call the decorator back and the conversation gets awkward fast, because by the time the paint starts failing, the other trades have been through the room three more times, the kitchen is in, and nobody can agree whose fault it is. The fix is stripping back and starting again.

Decoration is the last finishing trade. It looks like the straightforward part. It isn't.

Two things catch homeowners by surprise: the mist coat requirement on new plaster, and the fact that your decorator will visit four or five times across several weeks rather than arriving once and finishing the job. Both of these are simply how a new extension dries out and gets finished, not things your decorator is doing wrong.

What this guide covers

01Knowing when the plaster is ready
02The mist coat: why it's not optional
03Woodwork has its own sequence
04Why the decorator visits four or five times instead of once
05DIY or professional?
06What it costs
07Getting the quote right
08Snagging the finish

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