I always used a paint gun dedicated to primer alone. Primer gets into every nook and cranny and if you do not get your primer gunclean enough the paint solvents will loosen up the old primer and it will spray out in chunks on your paint job. You will not have to change the tips and needles all the time either. A good primer gun starts with a 1.8 tip, I used a 2.2the bigger tip will cause the primer to run and solvent pop if you are not careful, I would start with the smaller tip first and graduate to the larger if you think it is needed. More.....