Paint
Mist coat on new plaster: paint and water
- where
- thinned paint (mix) needed (L)
- area of new plaster (m²)
- number of mist coats
- spreading rate of the thinned paint (m²/L)
- paint (L)
- parts of paint
- water (L)
- parts of water
- water as a share of the paint (%)
Explain this equation
Works out how much thinned emulsion (mist coat) to mix for sealing new plaster, and how much of it is paint and water.
New plaster is very absorbent, so the first coat is thinned with water to soak in and bind. The total mix needed is area times coats divided by the spreading rate. The mix is split by the ratio: paint takes p parts out of p + q and water q parts. r shows the water as a percentage of the paint.
Coverage arithmetic, then splitting a mix by ratio.
Sense check: 40 m² at 17 m²/L needs about 2.35 litres of mix; at 5 parts paint to 1 water that is about 1.96 litres of paint and 0.39 of water.
Watch out: follow the paint maker's thinning advice. Vinyl and wipeable emulsions may not suit mist coats on new plaster.
Works out the paint and water for a thinned sealing (mist) coat of emulsion on new, dry plaster.
Enter your values to see the result.
Good to know
Data sheets set the dilution: Dulux Trade Vinyl Matt up to 1 part water to 5 parts paint for new or bare surfaces; Johnstone’s Covaplus up to 10% water on absorbent surfaces; Leyland Trade Vinyl Matt up to 1 part water to 9 parts paint. Use a contract or trade matt without vinyl-heavy sheen for the mist coat. The plaster must be fully dry (BS 6150 gives guidance on moisture before painting). New plaster soaks up paint, so the real spreading rate can be well below the “up to” figure: lower S if you know the surface is very porous.
Common mistake
Mist-coating with a vinyl silk or a paint the data sheet says not to thin, or thinning far beyond the stated limit, which leaves a weak, dusty film.
Limits
Quantities only. Drying, moisture content and conditions are covered on the technical page. Follow the specific product’s data sheet.
Sources
- Dulux Trade Vinyl Matt product data sheet (up to 17 m²/L; 2 full coats; seal new or bare surfaces with up to 1 part water to 5 parts paint; packs 1, 2.5, 5 and 10 L)
- Johnstone’s Trade Covaplus Vinyl Matt (up to 17 m²/L depending on porosity; seal absorbent surfaces with a coat thinned up to 10% with water; 1, 2.5, 5, 10 L)
- Leyland Trade Vinyl Matt technical data (about 14 m²/L; seal absorbent surfaces with up to 1 part water to 9 parts paint; 1, 2.5, 5, 10 L)
- BS 6150:2019 Painting of buildings. Code of practice
Results are calculations from the published methods and figures shown, and depend on the data you enter. Coverage depends on the product, surface and application. Always follow the paint and wallcovering manufacturers’ data sheets and BS 6150 / BS 8000-12. Check prices and VAT with your own records and HMRC.