I painted my mill with rustoleum in a gun. Been a while though, and I don't remember what I used to clean. I think it's an enamel though, so mineral spirits or paint thinner should work.
I was just thinking about getting rid of the gun. I'm not too impressed by how I cleaned it last time.
The SDS I just looked at says it has Naptha, MEK, and mineral spirits in it. I assume that means that mineral spirits should work as a cleaner. I don't know what "paint thinner" is these days.