- JulyBest month
- 132 daysWorkable days/yr
- 6 moSeason length
- 11–64%Rain-day odds
- Hillsboro by month
The next 10 days in Hillsboro
GOOD — a clean label day MARGINAL — one borderline check NO — hard fail or stacked flags
The rules this check uses
The engine scores every Hillsboro day against this table — typical latex-label numbers, with 35–50°F highs flagged for low-temperature formulas.
| 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 Hillsboro
| Month | Avg high | Avg low | Rain odds/day | Workable days | |
|---|---|---|---|---|---|
| January | 47°F | 34°F | 62% | 0 | |
| February | 51°F | 34°F | 57% | 0 | |
| March | 56°F | 36°F | 59% | 0 | |
| April | 61°F | 39°F | 52% | 4 | |
| May | 69°F | 45°F | 39% | 19 | |
| June | 74°F | 49°F | 28% | 22 | |
| July | 82°F | 52°F | 11% | 27 | |
| August | 82°F | 52°F | 12% | 27 | |
| September | 76°F | 48°F | 27% | 22 | |
| October | 64°F | 41°F | 47% | 11 | |
| November | 53°F | 36°F | 62% | 0 | |
| December | 46°F | 33°F | 64% | 0 |
The working season runs May through October — about 132 workable days a year. The edges are night-limited: label rules want overnight lows of 40°F+, and Hillsboro's nights only average that from May to October. Neighboring towns shift by a month or more — the Oregon comparison shows where Hillsboro sits.
Hillsboro has a real wet/dry rhythm: December brings rain on 64% of days versus 11% in July. When the calendar gives you a July-side window, the dry-before and dry-after rules nearly take care of themselves.
Related check: roof coating in Hillsboro — same 50–90°F chemistry, but roofs hit the dew point first and wind is a safety stop.
Climatology here is measured at Portland-Hillsboro Ap, Or Us (2.7 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.
For contrast, Gresham, OR’s exterior painting calendar sits one stop back on this route, and exterior painting in Medford, OR is next in line.
Hillsboro by the numbers
- Peak heat lands in August: 82°F average highs and 0 ninety-degree days.
- December bottoms the Hillsboro year: 46°F days, 33°F nights.
- Rain-day odds swing from 11% in July to 64% in December.
- Nights averaging 40°F+ run May through October.
- Annual workable exterior painting days: about 132 of 365.
Prep checklist
- Two clean days beat one perfect one: 24 h of dry cure and a 40°F+ night — July is Hillsboro's highest-odds month (27 days).
- Wash the wall and scrape everything loose; paint bonds to substrate, not chalk.
- Give washed siding 24 hours — north walls in Hillsboro can need double after a December-grade soak.
- Read the wall, not the app: an ir surface thermometer on sunlit siding shows 20°F+ over Hillsboro's reported 82°F.
- Spot-prime bare wood and bleed-through, then caulk the gaps on a touch-dry surface.
- Follow the shade around the house — never a wall in direct midday sun.
- 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 July lows near 52°F, Hillsboro's siding meets the dew point before the late news.
Pro moves for Hillsboro conditions
- Follow the shade around the house instead of the sun; siding that's hot to the touch will blister paint even when air temperature reads fine.
- Cold-rated acrylics that apply down to 35°F still need surfaces to stay above freezing for the next 36 to 48 hours, so check overnight lows.
- Dark colors can push a sunlit wall 30°F or more above air temperature; schedule dark accents for early morning or overcast days.
- Stop painting two to three hours before the evening dew point converges with air temperature, or the finish can develop dull spots and surfactant streaks.
- In Hillsboro, humidity typically bottoms out mid-afternoon; save the final topcoat for that window and use mornings for prep and priming.
Gear that saves a window
Heads up: product links on this page may become affiliate links when the program is enabled. See the affiliate disclosure.
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Canvas drop cloths
Grips ladders and won't shred like plastic.
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Extension pole
Second-story reach without moving the ladder every pass.
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Angled brush set
Control at trim, corners, and cut-in lines.
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Exterior paint + primer
One-coat hide on sound, prepped siding.
Frequently asked questions
What temperature can you paint outside?
Standard latex: 50–90°F with nights of 40°F+; low-temperature formulas are rated down to 35°F and the engine marks 35–50°F highs as MARGINAL for exactly that reason. Hillsboro's edge months live in that band — May averages 69°F highs over 45°F nights.
How many dry hours does exterior paint need before rain?
About 24 — a 0.05"+ shower inside that window streaks or washes fresh latex. Hillsboro offers those 24-hour dry runs most reliably in July (rain on just 11% of days); December is the gamble at 64%.
Why does dew ruin fresh paint?
Because a film that hasn't coalesced can't shed water: on cooling Hillsboro siding, dew flat-spots the sheen and drags surfactants out in streaks. It forms when the wall reaches the dew point — the engine wants a 5°F spread from 6–11 p.m. Finish 2 hours before sunset and latex gets its lead time.
Can you paint in high humidity?
The label limit is ~80% relative humidity, and it compounds: humid air slows the cure, which pushes wet film into dew hours. The engine flags 80–83% and fails beyond. In Hillsboro, the drier July air makes this a non-issue; muggy spells make it the day-killer.
What is surface temperature vs air temperature?
The forecast reports air; the label limits the wall. In direct sun a wall runs 20°F+ hotter — a 82°F Hillsboro July day can put a west wall past the 90°F ceiling by mid-afternoon. Follow the shade around the house and check the surface by hand or IR thermometer.
When does painting season end in Hillsboro?
The closing bell is the overnight floor. October is the last month averaging viable nights (41°F lows); after that, even warm afternoons sit on failing nights. Spring reopens around May from the same rule.
What humidity is too high for exterior painting?
Most acrylic paints apply well between 40 and 70 percent relative humidity. Above about 85 percent, water can't evaporate from the film, so drying stalls, sags develop, and surfactants rise as sticky brown streaks. Very low humidity under 20 percent causes the opposite problem — paint dries too fast to level. Check the hourly forecast for Hillsboro and aim for a falling-humidity afternoon.
What is the dew point rule for exterior painting?
Coating manufacturers require the surface to be at least 5°F above the dew point during application and initial drying. Closer than that, an invisible film of condensation forms on the siding, which ruins adhesion and can flash-rust bare metal. Practically, start after morning dew has fully evaporated and stop early enough that paint is set before the evening temperature drop.
How windy is too windy to paint outside?
Sustained winds above about 15 mph make brushing and rolling risky: paint skins before it levels, lap marks multiply, and dust or pollen embeds in the wet film. Spraying is far more sensitive — overspray drifts onto cars and neighbors above roughly 5 to 10 mph. Wind also accelerates drying, shortening your wet edge, so pick calm mornings for large walls.
Why is my fresh exterior paint blistering?
Heat blisters form when paint is applied to siding baking in direct sun — the surface skins over, then trapped solvent or water vaporizes underneath. Moisture blisters appear when paint goes over damp wood or when interior moisture pushes outward. Paint shaded sides, keep surfaces under about 90°F, and let rain-soaked siding dry a day or two before coating.
Exterior Painting glossary
- Dew point spread
- The gap between air temperature and dew point; painting standards call for surfaces at least 5°F above the dew point to prevent invisible condensation under fresh paint.
- Surfactant leaching
- Sticky brown or glossy streaks that surface when latex paint dries in cool, damp conditions; usually cosmetic and rinses off after the paint fully cures.
- Wet edge
- Keeping the boundary of just-applied paint wet so the next stroke blends invisibly; heat, wind, and low humidity shorten it and cause lap marks.
- Surface temperature
- The actual temperature of siding, which can run 20 to 40°F above air temperature in direct sun; paint labels' limits apply to the surface, not the forecast.
- Full cure
- The point when paint reaches final hardness and washability — typically 2 to 4 weeks for exterior acrylics — distinct from dry-to-touch, which takes only hours.
- Blocking
- Fresh paint sticking to itself where surfaces touch, like doors against jambs; worst in heat and humidity before the film fully cures.
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Guides
Climatology: NOAA 1991–2020 normals via PORTLAND-HILLSBORO AP, OR US (2.7 km from Hillsboro center, elevation 204 ft); live outlook by Open-Meteo.