Dew Point for Painting and Staining: Why It Matters and How to Check It

You can have a dry, sunny day and still ruin fresh paint with water. The culprit is dew, which forms whenever a surface cools to the dew point. Checking it takes a minute and explains many of the streaks, spots and cloudy finishes that people blame on the product.

What the dew point is

Air holds water vapor, and the cooler it gets, the less it can hold. The dew point is the temperature at which the air would be saturated (100% relative humidity). When a surface is at or below the dew point, water condenses on it: dew on the lawn, fog on a cold glass, moisture on a garage floor.

Unlike relative humidity, the dew point doesn’t swing much through the day. A dew point of 55°F in the afternoon is usually still around 55°F in the evening. What changes is the air and surface temperature, which falls toward it after sunset.

Why coatings care

• Latex paint: dew on paint that hasn’t cured can draw out surfactants (soap-like ingredients), leaving sticky, glossy or brown streaks. It usually washes off once the paint has cured, but it looks bad and can recur.
• Deck stain: moisture on the boards blocks penetration and can leave blotchy patches or a surface that stays tacky.
• Epoxy and other two-part coatings: moisture during curing can cause “blush”, a waxy or cloudy film that affects appearance and the bond of the next coat.
• Spray paint: in humid air near the dew point, the solvent’s cooling effect can pull moisture into the finish and turn it milky.

The 5°F rule

Coating data sheets and industrial painting standards commonly say the surface should be at least 5°F (about 3°C) above the dew point during application and while the coating sets. The margin allows for measurement error and for surfaces that are cooler than the air. The forecasts on this site use the same margin: an hour where the air is within 5°F of the dew point is flagged for painting, staining, epoxy, caulk and spray paint.

Check the surface, not just the air. Siding in shade, a concrete slab after a cold night or a metal railing can be several degrees colder than the air around it.

When dew forms

Dew forms most readily on clear, calm nights. With no clouds to hold heat in, surfaces radiate heat to the sky and cool faster than the air. Decks, roofs and car tops can drop below the air temperature and reach the dew point even when the air stays a few degrees above it. Cloudy or breezy nights produce much less dew.

Timing depends on the season:

• Summer: long days and warm evenings. Dew often forms late at night or near dawn, but in humid regions the dew point can be high enough to bring it on soon after sunset.
• Spring and fall: short days and cold nights. The temperature can fall to the dew point in the early evening, sometimes within an hour or two of sunset. That is why painters stop early in the afternoon at these times of year.

Morning dew matters too. Don’t start painting or staining until it has dried off, and remember that shaded sides of the house stay wet longest.

How to check it

1. Find the forecast dew point. The National Weather Service hourly graph at weather.gov shows temperature and dew point on the same chart. Where the two lines come close in the evening, expect dew.
2. Measure on the day. A digital thermo-hygrometer shows the air temperature, humidity and dew point. An infrared thermometer shows the surface temperature. Compare the surface temperature with the dew point.
3. Work out your stopping time. Most finishes need a few hours to become resistant to moisture. Finish early enough that they have that time before the surface reaches within 5°F of the dew point.

An example

A fall afternoon: high of 64°F at 3 PM, dew point 48°F, clear sky, light wind. Sunset is around 6:30 PM. The air is forecast to drop to 53°F by 8 PM, only 5°F above the dew point, and the siding on the north side will be cooler still. A latex paint that needs about four hours to resist moisture should go on no later than about 2 PM on the sunny sides and earlier on the shaded side. Starting at 3 PM invites streaks.

If dew hits fresh paint

• Don’t wipe it. Let it dry.
• Streaks from surfactant leaching can usually be rinsed off with water, or washed with mild soap and water, after the paint has cured for a few days. They may reappear once or twice.
• If the paint softened, wrinkled or washed off, let it dry fully, then sand lightly and recoat on a better day.

The exterior painting, deck staining and garage floor epoxy forecasts check the dew point for every hour of your work window, and for painting and staining they also warn if dew is likely within four hours of finishing.