The biggest, and most obvious thing we see, is painters who do not regulate the in-coming atomization air pressure correctly to the cup gun from a wall regulator. We see booths with wall regulators either maxed out at 90-130 psi, no regulator, unreadable gauges, cheater valves, undersized hose, or use of a regulator with minimal CFM capabilities.
Having a properly sized diaphragm type wall regulator to handle the volume you need is very important as some HVLP guns can require up to 15.2 CFM. We would recommend a minimum 30 CFM wall regulator and 5/16” or 3/8” air hose depending on length and gun CFM requirements.
Once you know you have a properly functioning diaphragm regulator with a readable gauge in your paint booth, adjust the air pressure 2-3 PSI more than the maximum inlet pressure for 10 psi (HVLP) at the air cap, with the gun trigger slightly pulled back. If equipped with an air micrometer built into the gun, make only minimal and final atomizing air adjustments.
If you are using an external air micrometer with a gauge at the bottom of your gun, then make sure this device is fully open prior to adjusting the wall regulator to 2-3 PSI above the recommended gun inlet pressure with the gun trigger slightly pulled back.
By adjusting atomizing air pressure without choked-down passages, proper air velocity is obtained through the spray gun. Thus allowing the air cap and fluid nozzle/needle combination to do their job of properly atomizing the coating stream into a mist, in an even and consistent manner.
If you instead leave in-coming air pressure very high, 100 psi at the wall, and cheat the pressure down with an air micrometer to 29 psi for instance, then you simply increased air velocity at less CFM and have not allowed the air cap time to properly atomize the coating stream.Thanks SATA