Rain-ready is not the same as dry to the touch
Latex paint forms its film as tiny particles fuse together, and cold or moisture can interrupt that even after the wall feels dry. Some products resist rain sooner: Sherwin-Williams' Latitude exterior latex is advertised to resist rain or dew in as little as one hour. That is a best case, not a promise for a cold evening. Oil-based paint is slower, with a recommended minimum of eight hours before rain above 70°F and longer when cooler.
In fall, the overnight low matters more than the afternoon high
Standard latex has long needed 50°F or warmer; some exterior paints can go on as low as 35°F, measured on the air, the surface and the paint. The common fall mistake, per the Sherwin-Williams exterior FAQ, is painting on a day above 50°F when the night drops below 35°F. The paint stops curing when it gets cold, and dew forms soon after sunset.
The same FAQ says the cooler it is, the earlier you should stop for the day. On a late-October day around Philadelphia, that can mean finishing by early afternoon so the last coat gets its dry hours before dew. Check the overnight low and dew point for the next two nights, not just the day you paint.
After rain, wait for the surface, not the clock
Sherwin-Williams suggests waiting at least a day after heavy rain, and the surface must actually be dry. Wood and masonry hold more water than aluminum or vinyl, and paint over a wet surface tends to peel. Its surface prep guide also says not to paint in fog or when rain is predicted. Shaded walls and the undersides of trim dry last. Already seeing bubbles? See why exterior trim paint blisters.
What rain or dew damage looks like
Streaks, runs, or shiny and dull patches on new latex: dew on uncured latex pulls a residue to the surface, called surfactant leaching. Aluminum siding is especially prone because it cools fast.
Glossy and flat patches on oil-based paint the next morning: dew on surfaces cooling after sunset, made worse by slow-drying gloss oil paint, per the exterior troubleshooting FAQ.
A whitish haze, called frosting: the paint got too cold or damp while curing. Sherwin-Williams says to rinse it with plenty of warm water from a garden hose, not high pressure.
Before anyone repaints, note the product, when the last coat went on and when the rain or dew arrived. The painter or manufacturer will ask.
Old paint and lead
In a home built before 1978, scraping or sanding old paint can release lead dust, which Sherwin-Williams warns is most dangerous for children and pregnant women. Under the EPA's lead renovation rule, anyone paid to disturb paint in a pre-1978 home, painters included, must work for an EPA-certified firm. Ask to see the certification before prep starts.
What to ask a painter before a fall exterior job
Which product and primer will you use, and what is its lowest application temperature?
How many dry hours does it need before rain or dew, and when will you stop each day?
How will you check that siding and trim are dry after rain?
What happens if the forecast changes, and is that in the written estimate?
For a pre-1978 house: is your firm EPA lead-safe certified?
Our contractor hiring checklist covers the rest, and you can request quotes on the painting services page.