Exterior paint is a chemical reaction. It cures by losing water to the air at a pace that depends on temperature, humidity, and airflow. Paint applied outside those windows looks fine on day one and fails in the first winter.
Temperature
Most premium exterior paints cure between 50°F and 85°F. Some (Benjamin Moore Aura) extend to 35°F with specific low-temp formulations, but the surface still needs to stay above the minimum overnight. April mornings in Massachusetts often dip below that — we rarely paint exteriors before mid-May for that reason.
Humidity & dew point
Paint cures by water evaporation. Above 85% relative humidity, the water can't leave — the film stays soft for hours and picks up dust. The dew point matters more than humidity alone: if the surface is colder than the dew point, moisture condenses on the siding under the fresh paint. We check surface temperature with a moisture meter before priming.
Rain window
Modern acrylic paints resist rain within 1–2 hours of application, but a heavy downpour on curing paint streaks the surface. We schedule with 48 hours of dry forecast on either side of a paint day and cancel rather than push through.
The sweet spot
Late May through late September in Massachusetts. Low-humidity September days are often our favorite — stable temps, low dew point, less pollen. October repaints work on wood and composite siding as long as overnight temps hold.