Andrew is correct about having clean DRY air to do quality work. A desiccant drier will eliminate water, a moisture trap will not. As air goes from higher pressure to lower, as at the nozzle of a paint gun, the temperature is dropped and most likely you will go below the level where condensation will occur = water droplets in you finish. Much more likely to occur in the humid summer than cold winter. There is a second method to reduce water vapor in you system, refrigerated drier. Expensive for the home guy but the normal practice for production shops. Most desiccant is sold with a color changing chemical in it so you can see when it is time to rejuvenate it. Just spread it on a cookie sheet in the oven at low temperature.