Can I paint outside in cold or damp weather?

Exterior painting has a narrow season in Britain and the limits are not arbitrary. Paint forms a film by evaporation or by chemical reaction, and both slow sharply as temperature falls and humidity rises.

What you'll need

• a thermometer
• a weather forecast including overnight temperatures
• moisture meter (optional)

Step by step

1. Read the minimum temperature on the tin.It is usually stated as a surface temperature, not an air temperature, and it is typically 8 to 10 degrees. Some winter-grade products go lower and say so explicitly.
2. Measure the surface, not the air.A north-facing wall in shade can be several degrees colder than the air, and it stays colder for much of the day. A cheap infrared thermometer settles this instantly.
3. Check the temperature is rising, not falling.Mid-morning to mid-afternoon is the usable window in cool weather. Starting at four o'clock means the paint film forms as the temperature falls.
4. Understand the dew point rule.As surfaces cool in the evening they reach the temperature at which moisture condenses on them. A paint film that has not formed by then gets water on and in it, which causes blooming, poor adhesion and a matt patchy finish.
5. Stop painting several hours before dusk.A common working rule is to stop by early afternoon in autumn and spring. It gives the film time to form before surfaces cool.
6. Check the substrate is dry.Timber and masonry hold moisture long after rain has stopped. Paint over damp substrate blisters and peels, sometimes months later.
7. Watch for overnight frost.A water-based coat that freezes before the film has formed is ruined and will not recover. Check the overnight minimum, not just the daytime forecast.
8. Avoid painting in direct hot sun too.The opposite problem. The surface skins before the paint has flowed out, giving brush marks, poor adhesion and blistering. Follow the shade round the building.
9. Expect much longer recoat times.At the low end of the range, a paint that recoats in four hours in summer may need a day or more. Do not judge by touch alone.
10. Consider a winter-grade product.Some exterior paints are formulated to apply and cure down to lower temperatures. They cost more and they genuinely work where standard products do not.

Tips

• Stop by early afternoon in spring and autumn. Painting into the evening is the commonest cause of a ruined exterior coat.
• Measure the surface temperature. A shaded north wall can be far colder than the air temperature suggests.
• Timber that looks dry can still be too wet to paint. Give it several dry days after rain.

Common mistakes

• Painting late in the day in autumn — The film has not formed when surfaces cool through the dew point. Condensation settles into the fresh coat, causing blooming, patchy sheen and poor adhesion.
• Painting over damp timber because the surface feels dry — The moisture is inside the timber and it has to go somewhere. It pushes the paint film off from behind, blistering and peeling within a season.

If it doesn't work

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