This is about as cheap as you can get.
I have an extra tank, the same size as the tank on my 3 HP compressor, 20 gallon.
I run a hose from the regulated out on the compressor, to a water trap/ filter mounted on the second tank. The air inlet on the second tank is on the top, pointing down, in the center. The air out to the paint gun is on one end of the second tank. I have a pressure gauge on the gun, that has a built in restrictor.
I use a $79.00 HVLP gun, from Home Depot.
This is how I set it up for painting. Regulate the air out of the compressor down to about 40 PSI, and put the second tank close to what I am painting. Adjust the pressure at the gun down to what I want, with the trigger pulled.
This setup allows me to paint more or less continuously, with a HVLP gun, and an old 3 hp Craftsman compressor. The compressor cycles on and off when I am painting, and spends more time off, rather than on.
This is my theory on why it works. We all know the air inside the compressor tank is hot, and has (non condensed) water vapor in it. When the hot air goes through the regulator, the pressure is reduced, and if you paid attention in high school chemistry, you know that allows the volume to increase, and the temperature to drop. The drop in pressure, and temperature causes the water vapor to condense in the air hose going to the second tank. The air also continues to cool a little more in that hose. The water trap can then catch the condensed water, and hold it. Any water that gets past the water trap, simply falls into the bottom of the second 20 gallon. Because the volume inside the second tank so large, compared to the air hose, there is no way any water in the second tank could be blown up to the outlet of that tank. The water just stays in the second tank.
It has been suggested that I move the regulator to after the second tank, because I would have more high pressure compressed air. Although that is true, the air in the second tank would be hotter, and still be holding on to the water vapor. The air has to cool for the water vapor to condense, and be trapped. The air expansion at the compressor mounted regulator cools the air, and a lot of the water vapor then condenses out before it gets to the filter/ water trap, and into the second tank.