I have a "trim paint gun" with about a 1 pint cup I bought from "Harbor Fright" many years ago. After having a lot of the same rattle can woes that have been described here, I've just about given up on buying those things. Like Jon, I've salvaged a lot of usable paint from cans that don't want to spray when I need a specific color, and the trim gun works well to apply it. The paint volume control and the fan spray knob makes it apply much more smoothly. Once the residual pressure is vented, poke a hole near the top of the rattle can to help the paint drain out of the bottom hole. Ditto on the acetone, but Mineral Spirits also works as a reducer on some types of paint. Various different drying time enamel reducers can be bought from automotive paint supply stores, too. Be sure to use a good pressure regulator, and an inline oil/moisture trap when using air from a hobby type air compressor- - - -that air can be pretty dirty at times.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!