High humidity can affect your interior or exterior paint job in more than one way. Water-based paint doesn't dry as quickly when the air is saturated with moisture, and that can result in drips and dust contamination. On the other hand, moisture gets trapped under solvent-based coatings, causing cracking or bubbles when the paint dries. Humidity is especially problematic when spraying -- it causes a milky appearance called clouding or blushing, which is caused when moisture is trapped in the finish. If you have to paint in humid weather, you may need the help of a heater or dehumidifier or -- in the case of solvent finishes -- a retarding thinner to slow the evaporation rate.
Because humidity slows evaporation of water-based finishes, you need to be wary of falling temperatures, which might cause water to condense on the wet paint.
- Refrain from painting outdoors late in the day when the relative humidity is high and temperatures are falling. The combination of high humidity and low temperature can result in the formation of dew, which can cause the paint to run. The best time to paint is in the late morning when temperatures are rising.
- Start by painting surfaces that have been warmed by the morning sun and are falling into shade. By the time you finish those areas, the sun should have warmed the rest of the surfaces you need to paint.
- Paint in thin coats; they dry more quickly. You may have to apply an extra coat, but you won't have to worry about running or prolonged tackiness.
- Provide ventilation to speed evaporation. Open windows to create a draft or run a fan. If the house has central air conditioning, run the system in "Fan" mode.
- Reduce humidity with a dehumidifier and/or raise the temperature with a space heater.
- Apply thin coats that dry quickly.
Many states limit the use of solvent-based paints indoors, but they are still in common use for exterior surfaces. In addition, you'll probably use a solvent-based finish when painting or finishing furniture.
- Scuff-sand the surface you're about to paint to make sure it's dry. You should be able to blow the sanding dust off the wood easily. If you can't, the surface is probably too moist to paint. Wait for it to dry. If you're painting metal or plastic, wipe the surface with a rag and inspect the rag for dampness.
- Apply paint in thin coats to ensure timely drying. Water won't make solvent-based paints run, but if it settles on the surface before the paint dries, it can leave marks.
- Add retarder to volatile spray finishes, such as lacquer or shellac, to prevent them from clouding. The retarder slows the evaporation rate, giving the moisture mixed with the finish material time to evaporate before a film develops and traps it.
- Regulate humidity in interior spaces, such as a spray room, by providing ventilation and heat and running a dehumidifier, if necessary.