Spray painting involves using an aerosol to apply paint mist to a surface. For the paint to stick to the target surface correctly, the surround temperature needs to be within the correct range. Humidity doesn't help successful painting when the moisture level is high. Instead, humidity makes it harder for spray paint to adhere properly and cure correctly.
Temperature Range
According to Tim Carter, author of "Ask the Builder" articles, the best spray painting environment has a temperature range of 50-90 degrees Fahrenheit. The better results tend to occur in the middle of the range, with little humidity and no wind. Humidity causes a moisture barrier between the paint and the target surface, and it is worse when the temperature gets significantly warm or rain is involved.
Effects of Humidity
When there is extreme humidity, the paint starts to get gummy in consistency and has difficulty hardening quickly. This condition does not work well for spray applications as the paint is more prone to coagulate and drip rather than stay in place. The moisture may also cause the paint to appear foggy rather than having a glossy-enamel look.
Longer Curing Time
Even if you manage to perform a clean spray application, humidity causes the surrounding temperature to impact the drying time of the applied paint. Where spray paint may dry completely in an hour or two in low humidity, the paint can take a couple of days to cure when humidity is high.
Solving Humidity
If you starting to see humidity effects in applied spray paint, such as fogginess or dulling, placing the painted part near a heated source can dry up the surrounding air before the paint has dried. Just make sure not to have the heat nearby when painting or so close that it touches the paint. Spray paint is flammable when fresh. Additionally, in some cases you may be able to polish a shine into the paint after it is dried, but it might be better to sand with fine sandpaper and repaint again for a clean coat look.
