The number on the tin is a surface temperature, not an air temperature, and it has to hold for the whole cure — not just while you are painting.
| Water-based acrylic | Oil-based | |
|---|---|---|
| Minimum surface temp | 50 °F (10 °C) | 45 °F (7 °C) |
| Low-temp formulations | 35 °F (2 °C) | — |
| Maximum surface temp | 90 °F (32 °C) | 90 °F (32 °C) |
| Ideal | 60–80 °F | 60–80 °F |
| Humidity | 40–70% | 40–70% |
Why the overnight low is the number that matters
Water-based paint dries in two stages. Evaporation takes hours; coalescence — the resin particles fusing into a continuous film — takes far longer and stops below the minimum film-forming temperature.
A coat applied at 65 °F on an October afternoon, with the temperature dropping to 40 °F overnight, evaporates fine and never coalesces. The result is a powdery, weakly bound film that looks perfect through the winter and comes off in sheets the following spring.
Check the overnight forecast, not the daytime one. That single habit prevents the most expensive exterior painting failure there is.
Dew point is the other trap
Paint will not bond to a wet surface, and dew is invisible on siding.
Surface temperature must stay at least 5 °F above the dew point while painting and while drying. On a clear autumn night, siding radiates heat to the sky and drops below air temperature — which is why an early-morning start on a beautiful day fails constantly.
Practical rule: start mid-morning, stop by mid-afternoon in autumn. In summer the constraint reverses and you start early to avoid the maximum.
Too hot is a real failure mode too
Above about 90 °F surface temperature, or in direct sun on a dark wall, paint skins over before it flows out. That gives brush marks, lap marks, poor adhesion and blistering as solvent trapped underneath escapes later.
A dark south wall in July can be 130 °F while the air reads 85 °F. Touch the siding; if you cannot hold your hand on it comfortably, it is too hot.
Follow the shade around the house, and remember the spread rate drops on textured siding regardless. Paint the east side in the afternoon and the west side in the morning. It is slower and it is the difference between a good job and a blistered one.
Humidity
Below 40% and water-based paint dries too fast to flow out, giving lap marks. Above 70% and it dries too slowly, staying tacky, collecting dust and insects, and risking a rain event before it is safe.
Rain resistance for most exterior acrylics arrives around 2–4 hours at good conditions and much longer in cool humid weather. "Dry to touch" is not "rainproof".
Wood moisture
Pressure-washed wood needs two or three dry days, not one. A moisture meter reading under 15% is the honest test; "it looks dry" is not, because the surface dries hours before the wood behind it.
Painting over wet wood traps the moisture, and it comes out later as blisters and peeling — the same symptom as painting too cold, from a different cause.
The season
Spring and early autumn in most of the US: mild days, mild nights, moderate humidity.
Late autumn is the riskiest time because the days can still be warm and the nights are not. That is exactly the combination that produces a film which fails in spring, and it is why so many exterior paint jobs seem to fail "for no reason" after one winter.
Get the conditions right and the gallon count is straightforward arithmetic — texture decides it, not brand, and primer only where the surface is porous.