Can I water down paint, and by how much?

Water in emulsion is the carrier: it evaporates and leaves pigment and binder behind. Add more water and you are not making the paint go further, you are making the film thinner, weaker and less opaque per coat. There are two situations where that is exactly what you want, and a great many where it is the reason a wall rubs off.

Step by step

1. Thin for a mist coat on new plaster or new board.This is the legitimate case. A thinned coat penetrates a porous surface and seals it, giving the topcoats a uniform base. Use a contract or matt emulsion, thinned roughly one part water to four or five parts paint, or whatever the tin specifies — and never a vinyl silk, which does not want to soak in.
2. Thin for spraying, to the equipment's requirement.Sprayers need a viscosity their pump and tip can handle, and the paint maker and the sprayer maker both state it. Follow those figures rather than guessing, and use a viscosity cup if the sprayer came with one.
3. Do not thin topcoats to stretch them.You are diluting the pigment that hides and the binder that gives adhesion and wash resistance. Two properly thinned coats do not equal two proper coats — you get a weaker film, worse opacity, and often a surface that chalks off on your hand later.
4. Add a conditioner rather than water for flow problems.If the paint is dragging, setting too fast or showing brush marks, a proprietary paint conditioner or flow extender is the right product. It extends the open time without diluting the solids, which is precisely what water fails to do.
5. Stir rather than thin if the paint seems thick.Emulsion settles heavily in the tin. A paint that seems unworkably thick is very often one that has not been stirred properly from the bottom, and stirring it thoroughly with a paddle or a mixer on a drill fixes it without adding anything.
6. Treat solvent-based paints as a different case entirely.Never add water to a solvent-based paint. If a solvent paint genuinely needs thinning, it takes the thinner named on the tin, in small amounts. Modern oil-based gloss is formulated to be used as supplied and thinning it usually causes runs and poor gloss.
7. Write down what you did.If you thin anything, note the ratio on the tin with a marker. Coats mixed at different dilutions dry to different sheens and opacity, which shows up as patches nobody can later explain.

Tips

• Add water a little at a time and stir thoroughly between additions. It is easy to over-thin and impossible to un-thin.
• Thinned paint runs and drips far more than you expect, especially overhead. Sheet up more than usual for a mist coat.
• If the tin gives a mist coat ratio, use it. Manufacturers formulate for their own product and some modern plaster-sealing emulsions want no thinning at all.

Common mistakes

• Watering down topcoat because a tin will not quite finish the room — The thinned area dries to a different opacity and sheen and shows for the life of the paint. Buy another tin — and box it with the first so the colour matches.
• Thinning a vinyl silk for a mist coat — Silk has more binder and does not penetrate porous plaster properly. Use contract or matt emulsion for the mist coat.
• Over-thinning a mist coat to a milky liquid — Too little binder means the coat itself is powdery and the topcoat bonds to dust. Stick to the stated ratio.

Questions people ask