How long should new render or mortar dry before I paint it?

Painting new render too soon fails in a way that looks like bad paint, so the paint gets blamed and a second coat goes on and fails as well. The cause is chemistry: cement curing releases free alkali, and alkali breaks down the binder in many paints. Time is the only cure, because what you are waiting for is not the surface to feel dry but the alkalinity to fall and the moisture to leave.

Step by step

1. Understand what you are waiting for.Two things at once — the render to finish curing and its alkalinity to fall, and the moisture in it to leave. A surface can feel dry to the hand while still being wet and strongly alkaline a few millimetres in.
2. Take the waiting time from the paint manufacturer.Every masonry paint states a minimum period for new cement-based surfaces on its data sheet, and they differ. Use that figure rather than a general rule, and treat it as a minimum rather than a target.
3. Add substantially for cold or damp conditions.Curing slows dramatically in cold weather and drying stops in damp weather. A wall rendered in October will not be ready on the same timescale as one rendered in June, and rain during the waiting period resets the drying.
4. Do not paint at all in the wrong conditions.Masonry paint has its own minimum temperature and its own dew point rules. Painting on a cold damp wall is a second way to get the same failure.
5. Test before you commit.A simple check is to tape a square of polythene tightly to the render and leave it a day or two. Condensation underneath means moisture is still coming out. For alkalinity, pH indicator papers on a dampened surface give a rough guide, and paint manufacturers sometimes specify a threshold.
6. Use an alkali-resisting primer where specified.Some systems allow earlier painting over a specified alkali-resisting primer or stabilising solution. That is a manufacturer's system, not a general workaround, and it does not remove the need for the render to be dry.
7. Watch for lime bloom before painting.White powdery or filmy deposits coming out of new render are the wall still discharging. Painting over an actively efflorescing or blooming surface traps the salts, and they push the paint off. Wait until it stops.
8. Brush down and remove dust first.New render carries loose fines and a dusty surface. Brush the whole elevation down before priming or painting, and deal with any laitance or a polished surface with light abrasion so the paint has something to grip.
9. Remember that through-coloured renders do not need painting.Monocouche and thin-coat silicone systems are coloured through their thickness. Painting them adds a maintenance cycle you were paying to avoid, and it can compromise the system's breathability.
10. Do not paint a lime render with an ordinary masonry paint.Lime render is chosen for its permeability, and most conventional masonry paints reduce that substantially. Limewash, a mineral silicate paint or a breathable coating designed for the purpose is the appropriate finish.

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