When to paint outside, and when not to

The four rules

1. Temperature between 10 and 30°C

Below 10°C, paint doesn't cure properly. Above 30°C, water-based paint dries too fast and brush marks lock in. Check the surface temperature too, not just air. Direct sun on a dark wall can be 15°C hotter than the air.

2. Humidity under 70%

Wet air slows drying. Paint stays tacky for hours, attracts dust and insects, may never fully harden.

3. No rain for 24 hours

Latex paint needs at least four hours dry before rain. Oil-based needs eight. Heavy dew also ruins curing, so finish at least six hours before sunset.

4. Low wind

Wind blows dust into wet paint. Above 25 km/h, you'll have grit baked into the finish.

Best painting seasons by climate

• Temperate (most of US and Europe): Late spring or early autumn.
• Hot dry (Australia, Mediterranean): April-May and September-October, early mornings.
• Humid subtropical: Find the driest stretch of late autumn.
• Tropical or monsoon: Wait for the dry season. Don't fight the weather.

The hot wall problem

If a wall has been in direct sun for hours, the surface can be 20 to 25°C hotter than the air. Paint flashes off the solvent in seconds and you get brush drag. Either paint whichever side is currently in shade, or start at 6am before the sun hits.

Prep matters more than weather

Even perfect conditions won't save a bad prep job. Wash with sugar soap. Scrape loose paint. Sand the glossy bits. Prime bare timber. Then check the forecast.

Today's call

Open Window Today's "Paint outside" card runs temperature, humidity, rain forecast, and wind through the rules above and gives you a clear yes or no.

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