Air filters clean junk out of the air.
Clean air is important because any junk in the air will mix with the aersol spray and deposit itself on whatever you’re spraying. This be anything from enough to ruin the finish on a car to something merely annoying when you’re looking at the finished piece.
Air filters generally have some kind of meaurement associated with the filter. This measurement indicates the smallest size of junk they catch. For instance a 5 micron filter catches dust particles as small as 5 microns in diameter.
Most “airbrush ready” compressors already come with an air filter of some kind, usually a 5 micron filter. Unless you need ultra clean air you won’t need a filter for smaller particle sizes. If you do need ultra clean air, you’re looking for HEPA filters and clean room technologies.
Industrial compressors, compressors intended for contruction work, don’t necessarily have air filters. Air tools don’t require very clean air. They just need a prefilter to remove the really big stuff and they’re happy.
In airbrushing there’re only two times you need to worry about air filters. When you don’t have one and when the one you have is too dirty or saturated.
Many filter elements can be cleaned with warm soap and water. You’ll need to check with your compressor’s manufacturer to be doubly sure though. Otherewise, you might need to replace the filter element.
Every filter unit will require a slightly different way to clean, or change the element, but for the most part, it’s an easy task that shouldn’t take more than 5-10 minutes once you know what you’re doing. In fact it uaually takes longer going through all the trouble to get the replacement than it does to actually change it out!