Exterior Painting Weather in South Lyon, MI: 10-Day Windows & Best Months

Q: What temperature can you paint outside?

50–90°F for standard formulas, 35°F+ for low-temp lines, and the wall itself must stay 5°F above the dew point. In South Lyon the practical range is set by nights: the 40°F overnight floor arrives around May and leaves after October.

Q: How many dry hours does exterior paint need before rain?

Plan 24 rain-free hours after the last coat; the engine fails any day that can't deliver them. With South Lyon's rain odds swinging from 36% of days in September to 57% in January, the strip above is mostly a search for that dry pair.

Q: Why does dew ruin fresh paint?

Fresh latex needs hours before it can take standing water; evening condensation gets there first on cooling siding. The check: air minus dew point from 6–11 p.m., 5°F or better. Humid January evenings in South Lyon are when GOOD afternoons hide failing nights.

Q: Can you paint in high humidity?

Up to about 80% daytime RH — above that, dry times stretch until the film meets the evening dew. 80–83% reads MARGINAL on the engine; more is a fail. Pair humidity with South Lyon's dew-point spread rule and paint mornings-into-early-afternoons in the humid months.

Q: What is surface temperature vs air temperature?

Two different numbers: air (what the app shows) and the wall (what the paint feels). Sun adds 20°F or more; evening radiational cooling subtracts. That's why the engine checks the 90°F top on South Lyon's hot afternoons and the dew-point spread after sunset — both are surface problems the air forecast hides.

Q: When does painting season end in South Lyon?

When nights stop clearing 40°F — in South Lyon that's typically after October, when average lows hit 41°F and falling. Low-temp formulas (35°F rated) buy a few extra weeks; the engine shows them as MARGINAL days before the hard close.

South Lyon gives you roughly 104 workable exterior painting days a year, concentrated May through October. The single best month is August, averaging 20 days that clear every check — highs of 82°F, lows near 59°F, and a 37% daily rain chance. Below: the live 10-day check and South Lyon's full month-by-month table.

GOOD — a clean label day MARGINAL — one borderline check NO — hard fail or stacked flags

The rules this check uses

The engine scores every South Lyon day against this table — typical latex-label numbers, with 35–50°F highs flagged for low-temperature formulas.

Typical label thresholds for exterior painting — the ruleset behind every South Lyon verdict above.
Check	Threshold	Why it matters
Air temperature	50–90°F (low-temp formulas from 35°F)	Standard latex wants 50°F+. Some low-temperature formulas are rated down to 35°F.
Overnight low	≥35°F during the first 24 h (≥40°F preferred)	Paint keeps curing overnight; a low under 40°F stalls standard latex.
Dry before	≤0.05" rain in the prior 12 h; watch back to 24 h	What fell before you start matters as much as what falls after — surfaces hold water invisibly.
Dry after	<0.05" rain for 24 h after	The make-or-break window: rain here undoes the work, not just delays it.
Evening dew-point spread	≥5°F from 6–11 pm	Surface should stay at least 5°F above the dew point; dew flat-spots fresh paint.
Daytime humidity	≤80%	Humid air slows evaporation, stretching dry times into the risky evening hours.
Wind	≤15 mph (brush only up to 20 mph)	Scored on the windiest working hour; the marginal band changes the method, not the day.

Always follow your product label — formulas vary. Treat the table as the consensus range across brands — the label in your hand is the final word.

Best months for exterior painting in South Lyon

How South Lyon months rank: temperature-rule days minus NOAA rain odds, from 1991–2020 normals. An estimate for planning, not a forecast.
Month	Avg high	Avg low	Rain odds/day	Workable days
January	32°F	16°F	57%	0
February	36°F	18°F	52%	0
March	46°F	25°F	48%	0
April	60°F	36°F	47%	2
May	71°F	47°F	46%	17
June	80°F	56°F	41%	18
July	84°F	60°F	38%	19
August	82°F	59°F	37%	20
September	75°F	52°F	36%	19
October	62°F	41°F	42%	11
November	48°F	30°F	46%	0
December	36°F	22°F	55%	0

Figure 104 workable days a year in South Lyon, spread across May through October. Shoulder months turn on the overnight rule: an afternoon at 71°F passes, but the 40°F night floor is what actually opens the season in May. Neighboring towns shift by a month or more — the Michigan comparison shows where South Lyon sits.

The rain odds swing hard across the year — 36% of days in September up to 57% in January. Season the plan accordingly: prep in the wet months, apply in the dry ones.

If the walls pass, the roof might too: roof coating in South Lyon uses the same film chemistry with tighter dew and wind limits.

Climatology here is measured at Ann Arbor U Of Mich, Mi Us (18.2 km away). Treat the monthly numbers as the neighborhood average; a shaded north-side deck runs colder and damper than any of them. The exact formula lives in the methodology.

South Lyon by the numbers

Peak heat lands in July: 84°F average highs and 0 ninety-degree days.
January bottoms the South Lyon year: 32°F days, 16°F nights.
Rain-day odds swing from 36% in September to 57% in January.
Nights averaging 40°F+ run May through October.
Annual workable exterior painting days: about 104 of 365.

Prep checklist

Find application day plus 24 dry hours with nights at 40°F+; South Lyon offers that pairing most often in August (20 workable days).
Wash the wall and scrape everything loose; paint bonds to substrate, not chalk.
Give washed siding 24 hours — north walls in South Lyon can need double after a January-grade soak.
An ir surface thermometer settles arguments: label limits bind the wall surface, which outruns South Lyon's air by 20°F+ in sun.
Spot-prime bare wood and bleed-through, then caulk the gaps on a touch-dry surface.
Sequence walls so you always paint in shade; midday sun skins latex before it levels.
Cut in with an angled brush set, roll a wet edge, and drop the sprayer plan over 15 mph.
Stop 2 hours before sunset: with August lows near 59°F, South Lyon's siding meets the dew point before the late news.

Gear that saves a window

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Extension pole
Second-story reach without moving the ladder every pass.
IR surface thermometer
Reads the wall, not the air — sun-baked siding runs hotter.
Canvas drop cloths
Grips ladders and won't shred like plastic.
Exterior paint + primer
One-coat hide on sound, prepped siding.

FAQ

What temperature can you paint outside?

50–90°F for standard formulas, 35°F+ for low-temp lines, and the wall itself must stay 5°F above the dew point. In South Lyon the practical range is set by nights: the 40°F overnight floor arrives around May and leaves after October.

How many dry hours does exterior paint need before rain?

Plan 24 rain-free hours after the last coat; the engine fails any day that can't deliver them. With South Lyon's rain odds swinging from 36% of days in September to 57% in January, the strip above is mostly a search for that dry pair.

Why does dew ruin fresh paint?