I bought one of the purple HVLP spray guns from Harbor Freight a while back, just to have as a throw away gun to do simple projects with. First project was a cart I made to set our Weber Q220 BBQ grill on under our covered porch. I took no pains with it, and only scuffed it up and sprayed it with single stage black urethane enamel I had left over from a car project. That supposedly crappy little $29 spray gun layed that paint down just as nice as my $400 Devilbis Tekna Pro Lite gun would have! I would not hesitate to do underhood paint or jambs the Horrible Freight gun!
