So just as you wouldn't want to paint during a downpour, you also wouldn't want to paint at the dew point. Dew also poses a serious problem when it forms on a recently painted surface. The moisture interferes with latex paint's coalescence process, making it dissolve and literally run off the house. Even surfaces painted late in the day that are dry to the touch by 5 p.m. can end up all over the shrubbery and sidewalk with the next morning's dew. Fortunately, there are methods for determining when you're entering the danger zone.
Some contractors invest in a psychrometer, a portable device for measuring the air's humidity.
If you know the humidity level and the ambient temperature, you can calculate the dew point with the help of a psychrometric table. If you're using a latex product, it's OK to paint over dew-coated surfaces, once the temperature rises 5 degrees above the dew point. That's because latex paints allow any remaining moisture to escape without harming the paint film. Still, the later in the day you paint, the more you risk problems with the next day's dew. If you're using an oil-based paint, wait until the surface is completely dry before painting.