This is my paint gun.
I use a 3 HP single stage compressor to supply air to it.
But I run air through this extra tank first.
What I do is locate the second tank close to what I am painting. I use the regulator on the air compressor to drop air pressure going into the second tank to about 40 PSI.
When the higher pressure air goes through the regulator, it obviously drops pressure. When you let a gas (air) drop pressure two other things happen. The volume of the gas goes up, and the temperature of the expanding gas cools.
This allows a "too small" compressor to keep up with my HVLP spray gun.