[5dogsisenuff]

Ok, Im just a little confused, what pressure do I set my compressor when I paint, do I set it a 90 psi, and then adjust my regulator at the gun, reason why im confused is that some guns say 12.8 at 40 or 9.4 at 50 or whatever what do they mean?

[Centerline]

The first number refers to the cubic feet per minute. The second is the pressure. So if you have an HVLP gun that recommends spraying with 12+ CFM at 40 PSI that means your compressor should have the capability of providing at least that CFM. If you're running an HVLP gun you should set the pressure at the gun because it's more accurate than at the compressor. The CFM delivery will depend on the size of your hose, the size of the tank, and the compressor's ability to provide that much air.

I'm sure some of our pros can explain it in more detail.

[5dogsisenuff]

Ive always left my compressor at 90 psi, and I just adjust my gun regulator, is that wrong?

[baddbob]

Adjusting your gun pressure at the gun is fine. The most important thing is to have a compressor and airline large enough to support the amount of air passing through the gun- this is where the cfm comes in. The cfm cubic feet per minute is the volume of air that will pass through the gun at your pressure setting. You can't spray a full sized gun with an airbrush sized compressor, an airbrush sized compressor may produce enough pressure but definately not enough volume. Now, if you were to fill a 2,000 gallon storage tank up with 90 psi using that little airbrush compressor the volume of pressurized air in the 2000 gallon tank would support a full sized gun for quite awhile then when the volume of air is gone the airbrush compressor wouldn't be able to fill the tank fast enough to keep spraying.
With your empty gun attached to the hose and adjusted how long does it take to get a 5psi drop in pressure with the trigger pulled? If there's no drop in pressure your compressor is large enough to support the gun, if there is a pressure drop after some time you better plan on taking a break during the shoot to let the compressor catch up.