An old painter's rule states: If it's wet enough so you can't hang out the laundry, wait and paint on a drier day. Painting in the rain, even indoors, puts the flawless results you want from all your hard work at risk. But, if you live in a rainforest or a soggy climate where the driest it gets on your days off is misty, you may have to improvise.
When to Worry
Dampness and high humidity cause paint to dry much more slowly and can even affect bonding as the drying paint adheres to the wall. Generally, latex or acrylic paint -- the water-based interior paint used for most walls and even trim -- responds best when the temperature is between 50 and 90 degrees Fahrenheit and the humidity is moderate. As the acrylic resin in paint dries, the water evaporates and the microscopic particles in the paint are drawn together and solidify to create a film with the color pigments in it. Higher humidity slows this process, and 100 percent humidity -- precipitation -- keeps the water in the paint so it doesn't form a hard film finish. This means your paint won't dry in time for a safe same-day second coat, and prolonged high humidity might cause problems with paint adherence to the wall and the eventual finish.
Take Evasive Action
Minimize the negative effects of a sudden shower in the middle of your carefully planned paint job by adjusting Mother Nature. Place fans throughout the space to create a breeze, and add a dehumidifier to extract the excess moisture from the room. Window-mounted box fans, securely positioned in open windows, will pull the paint fumes and some of that moist air out. Turn the heat or air-conditioning on to dry out the space faster. Plan your projects for the dry season and the clearest, moderately breezy day. Use rainy days to prep the room, and tackle the actual painting once the weather is dry. Allow extra drying time in humid weather, both between coats and for the final coat to harden and bond to the wall.
Exterior Painting Protocol
If it's raining and the walls you need to paint are outside, don't. Painting your house in the rain is a comical spectacle and a waste of time, labor and paint. The color will wash right off the walls or dilute to an unattractive fade, or the paint will fail when the film doesn't form properly or "cure" as a hard bond to the wall. In excessively high humidity, the paint may even take up moisture from the air, fail to dry, and instead slide off the painted surface. Smart house painters in temperate climates wait for summer to tackle the siding. That gives any wood or other absorbent materials time to dry out completely from winter snows and spring rains.