Painting Plasterboard for the First Time in a New Extension

The Unique Challenge of New Plasterboard

If you have recently had an extension built on your London property — whether a rear addition in Chelsea, a loft conversion in Fulham or a basement dig in Belgravia — you will likely be facing freshly skimmed plasterboard walls and ceilings. Painting these surfaces for the first time is straightforward when done correctly, but a surprising number of problems arise from skipping essential steps.

New plaster is highly absorbent. Apply a full-strength emulsion directly and the paint sits on the surface as a skin, poorly bonded and prone to peeling. The result is a decorating failure that requires stripping back and starting again — an expensive lesson.

Understanding the Mist Coat

The mist coat is the critical first step when painting new plaster. It is simply a diluted coat of emulsion that soaks into the plaster, sealing the surface and providing a key for subsequent coats.

The recipe is simple: Mix standard white matt emulsion with clean water at a ratio of approximately 70 per cent paint to 30 per cent water. Some decorators use a 60:40 ratio on particularly absorbent plaster. The consistency should be noticeably thinner than straight emulsion — more like single cream than the usual paint consistency.

What to use: A basic white matt emulsion is ideal. Avoid vinyl matt or silk emulsions for the mist coat, as the vinyl content creates the very barrier you are trying to avoid. A trade-quality matt emulsion without vinyl is the right choice.

What NOT to use: Do not use PVA as a sealer before painting. Despite persistent advice in DIY forums, PVA creates a plastic film on the plaster surface that can cause paint to peel. It has its place in construction (as a bonding agent for plaster), but not as a paint primer.

Timing: When Is New Plaster Ready

New gypsum plaster (the skim coat applied over plasterboard) needs time to dry before painting. The standard guidance is:

• Minimum two weeks in warm, well-ventilated conditions
• Up to four weeks in cooler months or poorly ventilated spaces
• The plaster should have changed from dark pink to a uniform light colour throughout

In London extensions completed during winter, we frequently find that new plaster is still damp weeks after skimming. Rushing to decorate over damp plaster causes efflorescence (white salt deposits), poor adhesion and potential mould growth. We test moisture levels with a meter before proceeding.

Dealing With Joint Tape and Screw Pops

Plasterboard construction introduces specific issues that do not occur with traditional solid plaster:

Joint tape visibility: Where plasterboard sheets meet, the plasterer applies jointing tape embedded in a thin layer of plaster. If the tape has been poorly applied or insufficiently feathered, it can show through the finished paint as a visible ridge. Before painting, inspect all joints carefully. Any visible tape edges should be skimmed over and sanded smooth.

Screw pops: The screws securing plasterboard to the framing can work loose slightly over time, particularly as the building settles. A screw pop appears as a small circular bump or depression in the surface. Before painting, check for screw pops by running a broad filling knife across the walls. Any that are found should be driven home slightly below the surface and filled with jointing compound.

Corner beading: External corners are protected by metal or plastic angle beads. These can be dented during construction. Check all corner beads and repair any damage with filler before decorating.

Application: Mist Coat and Topcoats

Applying the mist coat:

1. Protect flooring and joinery with dust sheets
2. Cut in around edges and corners with a brush
3. Roll the diluted emulsion onto walls and ceilings using a medium-pile roller
4. Work quickly and evenly — the diluted paint runs easily, so watch for drips
5. Allow to dry fully (typically four to six hours)

The mist coat will dry to a patchy, slightly chalky appearance. This is normal and expected.

First full coat: Apply undiluted matt emulsion using a roller with a medium-pile sleeve. Work in sections, maintaining a wet edge to avoid lap marks. In rooms with high ceilings — common in the grander extensions of Kensington and Mayfair — use an extension pole to reach upper areas without a ladder.

Second full coat: Allow the first coat to dry fully before applying the second. Two full coats over a mist coat provides excellent coverage and a durable, even finish.

Ceilings: Extra Care Required

New plasterboard ceilings are particularly unforgiving because light rakes across the surface, highlighting every imperfection. We recommend:

• Sanding the entire ceiling lightly with 120-grit paper before the mist coat
• Using a bright work light held at a low angle to inspect for defects
• Applying paint in one direction, working away from the primary light source
• Using a quality flat matt rather than any sheen, which amplifies surface irregularities

Choosing Your Finish Paint

For newly plastered walls in a London extension, the choice of finish paint depends on the room's function:

• Living areas and bedrooms: A quality matt emulsion from Farrow & Ball, Little Greene or Dulux Trade provides a sophisticated finish
• Kitchens: A durable matt or eggshell that can withstand occasional wiping
• Bathrooms: A moisture-resistant formulation designed for humid environments
• Hallways: A hardwearing matt or soft sheen that can handle scuffs from daily traffic

Common Mistakes We See

Working across London extensions in Westminster, Battersea and beyond, we regularly encounter these avoidable errors:

• Painting over damp plaster, leading to peeling within months
• Using vinyl silk for the mist coat, creating adhesion failure
• Ignoring joint tape defects that become glaringly obvious after painting
• Applying only one topcoat, resulting in patchy coverage

With proper preparation and patience, painting a new extension is one of the most satisfying decorating projects. If you would prefer professional results, we are happy to provide a quotation.