How early is too early to start exterior painting once it starts cooling off?

[JimmyMechanic]

Planning to repaint the trim and siding before it gets too cold, but I'm second-guessing my timing. Mornings are already getting dewy here and I don't want paint failing because the surface wasn't fully dry or the temp dropped too fast overnight.

For those who've done fall exterior painting — what's your actual cutoff, temperature and humidity-wise? Do you wait until midday no matter what, or is there a point in the season where you just call it and wait until spring? Also curious if anyone's had bad luck with paint not curing right because they pushed it too late in the year.

[HankBriggs]

The number that actually matters more than the calendar is dew point, not just air temperature. Paint needs the surface temperature to stay at least about 5°F above the dew point during application and while it's drying — if it doesn't, moisture condenses into the film before it's cured, which is what causes the blotchy/hazy finish or adhesion failures people get blamed on "painting too late in the season."

Practical guidelines:

• Check your specific paint can — most modern exterior latex is rated down to 35-50°F depending on the formulation, but that's a minimum, not a target. Rising temps in the 50s and climbing is a safer zone than a can that just barely clears the minimum.
• Don't trust "looks dry" on morning dew — wait until the surface actually tests dry (touch test, or a moisture meter if you have one), which in fall usually pushes your start time to mid-morning once dew has fully burned off, shortening your working window.
• The riskier moment isn't the application itself, it's the first 24-48 hours after. If a hard frost or heavy dew is forecast overnight before the film has fully set, that's when you get cracking or poor adhesion even if the daytime application conditions were fine. Check the overnight low for at least the next day or two before starting a section.
• General cutoff a lot of pros use: once nighttime lows are consistently dropping into the 40s, exterior latex painting starts getting genuinely risky, and once you're regularly seeing overnight temps near freezing, it's time to wait for spring regardless of how mild the daytime highs still feel.

If you're already seeing heavy morning dew, you're in the shoulder season — doable, just plan for a later start each day and keep an eye on the overnight forecast rather than only the daytime high.