In rain, dew, or other moisture conditions, the main issue is how damp the surface is, not the air. Yes, you can paint in humid conditions. Run your hand across the surface. Does it feel wet? Even if it doesn't feel wet to the touch, it probably is.
Hinges on Water and Solvent Drying TimesStill, this may not necessarily bar you from painting. After all, unless you live in an arid climate, your air is humid. Air naturally holds moisture.
Mark Knaebe a Chemist at the USDA Forest Products Laboratory provides good insight as to how humidity can affect a painting surface.
It comes down to a race between drying times of water in the paint and the solvents in the paint. Which will dry first?
The water needs to evaporate at a rate as fast or faster than the solvents. If the water interferes with solvent-drying, then the paint cannot form a hard surface.
"When it is too humid," writes Knaebe, "water cannot evaporate and the solvents may evaporate first, causing the paint to cure while still in a water-filled state. You cannot recover from this type of disaster.
This does not just affect water-based paints. Knaebe notes that oil-based paints are prone to the same conditions.
Acceptable Humidity LevelsOptimal humidity levels for exterior painting, says Karl Crowder, Crowder Painting from Colorado Springs, Colorado, tend to be in the 40-50% range or lower.
Levels of 70% or greater will "drastically slow drying and curing," but again will not prevent you from painting.
What About Interior Painting?Interior painting is a different matter because the climate can be controlled.
For one, you've got protected surfaces. This will reduce dampness on the walls.
More importantly, HVAC (air conditioning or heat) or dehumidifiers running within the house can void the house of excessive moisture.