I don't know much about Harbor Freight spray guns. They might be OK, but I am pretty sure they are not real high quality. The better spray guns like my Anest Awata LPH400 and Sata Jet are quite expensive, but lay down the paint very smoothly. If you colorsand your painted parts like I do, I suspect a cheaper spray gun would work OK, it would take more sanding to smooth the paint, before compounding it. If you don't want to colorsand, cheap spray guns don't atomize the paint very well and you will get more orange peel. If you do colorsand the paint to get a nice smooth finish, you need 3 coats of paint to make sure you don't sand through the paint even though the sandpaper is quite smooth (1500 to 2000) grit. Colorsanding really makes a paint job look better.
Rusty Nelson