What is the best way to control relative humidity in paint applications?
This depends on what you are spraying and the surface you are spraying on. There are a lot of waterborne paints that require low humidity to dry out the air and help them dry faster. It is best to reference the technical data sheet of the coating you are spraying. It will specify the desired humidity level, and there are ways to adjust the paint to react better to that humidity.
If you are spraying waterborne paint, the ideal temperature in your booth is 70 to 75 degrees Fahrenheit, or 5 degrees above ambient temperature. When your booth is above that outside temperature, the burner will intermittently turn on to burn out the humidity and lower the overall humidity level in the booth.
In hot, dry climates like Arizona, painters experience the opposite problem. When it is too dry, they will use misting devices in the intake ductwork to ensure the paint does not dry too fast.
Solvent-based coatings dry well on their own regardless of the humidity, and in some cases will dry faster with a higher humidity. Regardless, it is good to have a way of monitoring humidity in the paint booth.