When the outdoor painting season starts and ends in Madison

What Madison's climate normals say

The Wisconsin State Climatology Office at the University of Wisconsin-Madison publishes Madison's climate normals for 1991 to 2020, which are 30-year averages. These are the months that matter for work outdoors.

Madison averages 37.13 inches of precipitation a year, and June is the wettest month. Look at the gap between highs and lows. In April and October a normal afternoon is fine for painting while a normal night is not, and that gap is where most outdoor paint problems begin.

The temperature rules paint makers set

Latex paint dries into a film as its particles fuse together, and cold interferes with that. Diamond Vogel's technical bulletin on exterior latex says it shouldn't be applied when the surface or the air is below 50°F, and that the temperature shouldn't fall below 50°F for at least eight hours afterward.

Some products are made for colder work. The same bulletin says certain exterior latex paints can go on at 35°F, provided the air and surface stay at or above 35°F for the next 24 hours. Dunn-Edwards, another paint maker, says its latex paints form a film properly down to 40°F, some of them to 35°F, and that the first 48 hours are the most critical. Dunn-Edwards adds that a low rating doesn't account for surface temperature, humidity or dew.

Why the nights matter more than the afternoons

Dew is the usual culprit. Dunn-Edwards notes that in spring and fall, when evening temperatures drop, condensation can form on freshly painted surfaces. The water builds up, runs down the wet film and leaves streaks.

Diamond Vogel's guidelines are specific. Hold off if the air temperature is less than 5°F above the dew point, if moisture is forming on the surface or on nearby surfaces, if humidity is 90 percent or higher and the temperature is expected to fall, or if rain is forecast within eight hours of finishing.

That's why shoulder-season painting happens at midday. Dunn-Edwards suggests applying paint between 10 am and 2 pm, so surfaces can warm up first and the paint has time to set before dew falls.

Heat has a limit too. Diamond Vogel warns that paint forced to dry quickly, in direct sun, wind or temperatures above 95°F, can dry before the film has formed properly. Madison's normal July high is 82.1°F, so extreme heat is a handful of afternoons rather than a season, but it's why a shaded wall is a better bet on a hot day.

Putting the normals next to the rules

Normal lows are averages, so about half of nights in a given month run colder than the figure. Even so, comparing them with the two thresholds above shows how narrow the window is.

Read it as a guide, not a calendar. A 50°F rule is comfortable from June through August, tight at the edges of summer, and mostly out of reach in April, May and October. The 35°F products stretch the season, but they still need the surface and the air to cooperate.

Spring: waiting out the last freeze

The Weather Guys, a weather column from the University of Wisconsin-Madison, report that the median date of Madison's last 32°F reading is May 2, based on 1981 to 2010 records from the State Climatology Office. Half of all years still see a freeze after that. By May 20 the chance has dropped to 10 percent, and on April 18 there is a 90 percent chance that a later freeze will come.

Freezes aren't the only spring limit. April averages 3.78 inches of precipitation and May 4.10, and exterior wood that is still wet from snowmelt and spring rain isn't ready for a finish. A dry week after the thaw matters as much as the temperature.

Fall: a shrinking window

At Madison's airport, the average first 32°F night is October 1, according to The Weather Guys. The old downtown station, which overlapped the airport's records from 1940 to 1963, averaged around October 20, so your own yard can differ by a couple of weeks.

Either way, the pattern is the same. October's normal high is 59.6°F and its normal low is 39.8°F, so by then the evening, not the afternoon, is the limit. By November the normal low of 28.7°F is below even the 35°F floor that some products allow. Our fall painting checklist covers what to finish before that happens.

What that means for doors, trim and decks

Small exterior projects fit this climate better than big ones. A door, a garage door or a run of trim usually takes one to three days, and we schedule exterior work around dry weather, so a project can wait for a good stretch instead of racing a cold front.

Prep takes weather too. Outdoor surfaces are cleaned, sanded, repaired and primed before any paint or stain goes on, and each step needs a dry surface. That is why a job that takes one to three days can need a longer stretch of decent weather around it.

• Doors and trim need dry surfaces and a forecast without rain for the first day or so, because the fresh film is most vulnerable early. See our guide to painting exterior trim.
• Decks need dry wood and steady weather across the whole job. Our post on the best time to stain a deck goes deeper.
• Garage doors and entryways catch full sun on one side of a house, so the time of day matters as much as the month.

Winter wears all of this down, which is why spring inspections matter. Our post on freeze-thaw damage explains what to look for. For entry doors, our post on painting a fiberglass door lines up one door maker's temperature window with this season, and painting a steel entry door adds the dew point check metal needs.

How to plan around the forecast

1. Read the forecast for the day you plan to paint and the two days after, not only the high.
2. Compare the overnight low with the minimum on your paint's label.
3. Check that the afternoon air temperature is at least 5°F above the dew point.
4. Start once the morning dew has burned off, and plan to finish with hours of warm sun left.
5. If rain is forecast within eight hours of finishing, wait.

If you'd like help with a door, trim or deck, ask for a free estimate. George will talk through the job and send a written quote with a start date, and we work throughout Madison, including the city itself and the towns around it.