Hygrometer
It is 63°F at four in the afternoon. The tin says apply above 50°F and you are comfortably clear, so you carry on and finish the last board at half past five, pleased with the light.
By nine the deck is wet. Not with rain — the sky has been clear all evening. With something that came out of the air onto a surface that got colder than the air did.
The coat you laid down at five has spent the night with water sitting on it, and it is going to tell everyone about that for a year.
Why the label does not warn you
The temperature range on the tin is about film formation — the chemistry of the coating coalescing into a continuous layer. Below the minimum, that process fails. It is a real constraint and the manufacturer is right to state it.
But it says nothing about what happens to a not-yet-cured film four hours later, and that is a completely different question with a completely different number behind it.
Coatings need water to leave. Dew is water arriving. The label governs the first and is silent on the second.
The number that actually matters
Surfaces on a clear night radiate heat to the sky and end up several degrees colder than the air. When they cross the dew point, water condenses onto them.
So the question is not "is it warm enough" but "how many hours until surfaces reach the dew point?"
| Dew point | Spread | Dew forms | Latest sensible finish |
|---|---|---|---|
| 40°F | 23°F | Probably not at all | Late afternoon is fine |
| 50°F | 13°F | Late, if the sky stays clear | Early afternoon |
| 58°F | 5°F | Soon after sunset | Late morning |
| 61°F | 2°F | Before sunset, on any still evening | Do not start |
Identical thermometer readings. Four completely different working days.
The working rule
- Check the evening dew point, not the afternoon temperature. The forecast shows both, and only one of them matters here.
- Work out the spread. Air temperature minus dew point. Under about 9°F and dew is likely soon after dark; under 5°F and it is close to certain on a clear, still night.
- Stop four to five hours before sunset for a latex coating in mild conditions — more for slow sealers, more on a humid day, more for a heavy film.
- Watch the sky. Clear and calm is the condition for dew, not for safety. Overcast and breezy is actually the more forgiving evening.
The specific failures
- Surfactant leaching. Latex paints contain water-soluble surfactants. Water landing on a partly cured film pulls them to the surface as brown or clear streaks — often mistaken for dirt or sap.
- Blushing. A milky haze in the film where moisture got trapped as it cured.
- Gloss loss and poor adhesion. Water at the surface during coalescence interferes with the film forming properly, and the result is a coat that scuffs and peels early.
- Simply not curing. With no headroom left in the air, evaporation stalls and the film stays soft far longer than it should.
The morning question too
The same arithmetic runs in reverse at the start of the day. A surface that was wet with dew overnight is not dry the moment the sun hits it — the water has to evaporate, and in shade that can take until mid-morning or later.
Starting on a surface that looks dry but is not is the other half of this problem, and it produces the same failures. If dew formed overnight, give the surface real time, and check the shaded parts by hand rather than by eye.