I also use a Grex and love it. I use about 28 lb. of pressure and the number 3 tip. Your primer has to be smooth. I use 3600 polishing cloth . Paint should be the consistency of 2 percent milk. i only use lacquer paints. A trick i use is to spray 876 lacquer thinner as the last coat. If you have a problem email or pm me. You have to have patience and practice. If you can join a club do it. Go to shows - ask questions that is how i started .
Hope this helps. Castrol Super Clean can be your best friend when you are starting out.
Bob's Paint, etc.