I usually throw a half inch of lacquer thinner in the bottom, shake it good with my finger over the vent hole on top of the cup, and spray 10 seconds. Then I pour it out, and wipe the gun out. Then I repeat that a second time.

Basecoat urethanes tend to stick inside the gun, leaving a film. Metallics and flake also like to hide inside the gun. If you are going to shoot clear, or some other paint that would let small amounts of contaminants show, then tear the gun apart and scrub it out with brushes. After that, run some thinner through the gun again to make sure that there isn't anything floating around loose in there.

There is nothing more frustrating than a nice black paint job, that got a couple specks of metallic in the clearcoat!

I use one of my guns just for clear, to minimize the risk.

Single stage paints are nice, since you don't have to worry about dirt in the clearcoat, or sanding through the clear. The drawbck is that it has to be a solid color. Metallics and pearls colors will not be able to be sanded or buffed, unless they have a clear coat on top, since you would be sanding and disturbing the metallic!