How do I mist coat new plaster?

Fresh plaster is highly porous and slightly alkaline. Full-strength emulsion applied to it dries as a film sitting on the surface rather than keying into it, and it comes off later in large flakes.

What you'll need

• matt emulsion for the mist coat
• water and a mixing bucket
• roller and brush
• dust sheets

Step by step

1. Let the plaster dry completely.It goes from dark brown or pink to a uniform pale colour. Any darker patches mean it is still holding moisture, particularly in corners and behind radiators.
2. Allow enough time.Typically several days to a couple of weeks depending on thickness, ventilation and season. Backing coats and thick repairs take much longer than a thin skim.
3. Ventilate rather than heat.Air movement dries plaster properly. Forcing it with a heater dries the surface while the depth stays wet, and can craze the finish.
4. Prepare the surface.Knock off any plaster nibs with a scraper, brush the wall down, and fill any small defects. It is much easier before the mist coat than after.
5. Use ordinary matt emulsion.Cheap contract or trade matt is ideal. Do not use vinyl silk, one-coat or paints containing PVA or additives — vinyl and silk finishes form a film rather than soaking in.
6. Thin it by 20 to 30 per cent.Roughly three parts paint to one part water, mixed thoroughly. It should be noticeably runnier than normal emulsion but not like water.
7. Never use diluted PVA as a primer.It seals the surface into a glossy film that emulsion will not bond to. It is one of the most persistent bits of bad advice in decorating.
8. Apply it quickly and evenly.It goes on thin and dries fast. Work in manageable sections, keep a wet edge, and expect it to look patchy — it is meant to.
9. Expect it to be messy.Thinned paint runs and spatters. Dust sheets everywhere, and a roller shield if you have one.
10. Let it dry, then look for suction problems.Areas that dried noticeably faster and duller are more porous and may need a second mist coat before the topcoats.
11. Apply two full coats over it.Full-strength emulsion, cutting in first then rolling. The mist coat is not a finish coat and it will not look like one.

Tips

• Contract matt is the ideal mist coat paint and the cheapest option in the shop.
• Never use PVA on plaster you intend to paint. It seals the wall and guarantees the paint will peel later.
• Patches that dry faster and duller have higher suction. Give them a second mist coat before the topcoats or they will flash through the finish.

Common mistakes

• Painting new plaster with full-strength emulsion — The plaster is too porous and the paint too thick to penetrate. The film dries on top with almost no key and peels away later, often taking a skin of plaster with it.
• Sealing the plaster with diluted PVA first — PVA forms a glossy, non-porous film. Emulsion over it has nothing to grip and it peels, often in one continuous sheet.

If it doesn't work

Questions people ask