What is a Mist Coat?
A mist coat is the thinned first coat of paint used on new plaster or very porous surfaces. The main goal of this is to cover the surface and decrease its absorption capacity.
When painting freshly plastered walls, a mist coat is crucially important. The first coat will seal the plaster and decrease its absorption, promote better adhesion and smooth finish for the following coats.
Why is the Ratio Important?
If the mist coat is too thin, it may not provide enough coverage and sealing. On the other hand, if the coat is too thick, it may not stick to the surface properly and may peel or flake off in the future.
Manufacturer Recommendations
The first reference for a mist coat ratio should always be the paint manufacturer’s instructions. Here are some examples:
- Leyland Contract Matt: In most cases, they recommend a 10% water dilution (9 parts paint to 1 part water).
- Dulux Trade: It is usually recommended to dilute the paint with 25% water.
A rather common recommendation is to take a 50/50 paint-to-water solution, but this is not without its disadvantages:
- The mix can be too thin, and the plaster will absorb it right away, which may lead to some patchiness.
- Getting good coverage might be a problem, and you may need to apply more coats.
Common Ratios and Considerations
Mist coat ratios that are most often recommended are between 70/30 and 80/20 (paint to water). Some factors that may influence your decision are:
- Emulsion Type: Non-vinyl matt emulsions (often labeled as “contract matt”) are preferred as vinyl-based paints do not penetrate plaster well.
- Plaster Porosity: If the plaster is very porous or dry, it may be necessary to dilute the paint a bit more in order to get it to penetrate the plaster.
Best Application Practices
- Thorough Mixing – Stir the paint and water well. A plastic spatula is used to hand mix the paint to get an even consistency.
- Even Application – Roll on it liberally but even. Try to blend the edges and avoid overlap lines as much as possible.
- Accept Some Imperfections – The first mist coats are usually not perfect and you can see some irregularities. These imperfections can be rectified and then resolved in subsequent coats (see below).
- Avoid Premature Cleaning – Do not wipe freshly painted mist coats with damp cloths this can peel off the thin layer.
- Fix Imperfections After Drying – Fill and sand out all the imperfections that the mist coat has shown. If the area is dry, apply another coat of mist coat to the repaired area.
When to consider a second Mist Coat
If the first coat looks uneven or the plaster is very absorbent, a second mist coat (perhaps less diluted) can help to even out the coverage and sealing. If the defects that were exposed by the first coat have been filled and sanded, then a second mist coat or a half second mist coat will be sufficient to provide a good surface for further painting.