If you are using the HF "Purple" 43430 gun. there are some copies of that POS that will need lots of pressure. The one I had - 45+psig at the inlet. Then you are also painting everything in sight/on site with overspray. I sold mine and got a good gun for primers as well as a great gun for BC/CC.

These HF guns can and do work well once you have them figured out, but at the cost of lots of paint product not sprayed on the car.