DryWindowBest Weather for Exterior Painting: Temperature, Humidity and Dew Point
Most exterior paint failures – peeling, blistering, a chalky or patchy finish – are not the paint's fault. They come from painting at the wrong time of day, in the wrong weather. Here is what the weather needs to do, and why.
The four numbers that matter
A normal forecast tells you whether it will rain. For painting, that is only one of four things you need to know. The others are air temperature, relative humidity and the dew point – and the dew point is the one most people never look at.
1. Temperature
Most water-based masonry and exterior wood paints list a minimum application temperature of around 8–10°C, for both the air and the surface. Below that, the binder in the paint cannot coalesce properly into a continuous film. The paint may look fine for a week and then begin to crack or wash off. Oil and solvent-based gloss paints tolerate slightly lower temperatures, but they dry much more slowly in the cold, so they stay soft and vulnerable to rain and dirt for longer.
Too hot is a problem too. On a sunny wall the surface can be 15–20°C warmer than the air. Paint applied to a hot surface dries before it levels, leaving lap marks and brush marks, and can blister as trapped moisture or solvent expands. A practical upper limit for most products is around 30°C air temperature, and you should avoid painting a wall while the sun is directly on it.
2. Relative humidity
Water-based paint dries by evaporation. If the air is already close to saturated, evaporation slows down dramatically. Above about 85% relative humidity, drying times can double or triple and the film can stay tacky into the evening, which is exactly when dew starts to form. DryWindow uses 85% as the limit for standard exterior paints and a tighter 80% for stains and coatings that are more sensitive.
3. Dew point
The dew point is the temperature at which moisture in the air condenses into liquid water. If a surface cools to the dew point, it gets wet – even if it has not rained. Professional coating specifications almost always require the surface temperature to be at least 3°C above the dew point during application and while the coating dries. DryWindow applies this rule to every hour it checks. Our dew point guide explains this in more depth.
4. Rain – during and after
Paint needs a rain-free period after application, not just during it. Many water-based exterior paints are "shower-resistant" after one to two hours in good conditions, but that figure is quoted at around 20°C and 50–65% humidity. On a cool, damp British autumn day, the same paint may need four to six hours or more. DryWindow therefore asks for six hours rain-free after a water-based paint job and twelve hours after an oil-based one.
Choosing the time of day
In spring and autumn, the best painting window in the UK is often surprisingly short: roughly from mid-morning, once overnight dew has evaporated, until mid-afternoon, so that the paint has a few hours to dry before the temperature falls and dew returns in the evening. In summer the window is wider, but you will want to follow the shade around the building – paint the east side in the afternoon and the west side in the morning.
Starting too early is one of the most common mistakes. A wall that was wet with dew at 8am can still be damp at 10am even when the air feels dry. Touch it with the back of your hand and, if in doubt, wait another hour.
Wind
A light breeze helps paint dry. Strong wind does not: it blows dust, insects and leaves into the wet film, makes spraying impossible, and can dry the surface skin too quickly while the paint underneath is still wet. DryWindow flags hours where the wind is stronger than about 19 mph (30 km/h) for brush-and-roller work.
A practical checklist
- Check the forecast for the day of painting and the following night.
- Look for at least 4 hours of air temperature above 10°C, humidity below 85% and no rain.
- Make sure the temperature stays at least 3°C above the dew point while you work.
- Allow at least 6 rain-free hours after you finish (12 for oil-based paints).
- Do not paint surfaces in direct hot sun or that are still damp from dew or rain.
- Read the data sheet for your specific paint – some low-temperature formulations allow application down to 5°C or below.
Surfaces that need extra care
New render and concrete must cure fully before painting; this can take weeks, and the alkalinity of fresh cement can damage some paints. Timber should have a moisture content below around 18% – after a wet week it may need several dry days before it is ready. Metal gutters, railings and garage doors heat and cool quickly, so they are the first surfaces to reach the dew point in the evening and the first to get too hot at midday.
How DryWindow helps
Instead of checking four different numbers for every hour of the week, enter your location and choose "Exterior paint – water-based" or "Exterior paint – oil-based". The tool scans the 7-day hourly forecast, rejects any hour that breaks one of the rules above, checks the rain-free period after each possible start time and ranks the windows that remain. The colour chart shows you at a glance which days are worth planning around.