Think of the air volume adjustment like a sink faucet. It adjusts the amount of air flowing through the gun without necessarily affecting the pressure. More air can mean thick coats of paint but also more messy overspray; too little can mean thin, grainy coats of paint. Start with the adjuster in the middle. Spray some paint on a test panel and fine-tune the adjustment until you are satisfied. Note that you may need to make small corrections to the air pressure regulator as you adjust the air volume.