Most air compressors have a maximum static air pressure of between 100 to 120 PSI, 689.5 to 827.4 Kpa, at which point the air compressor pump turns off. This amount of air pressure is way too high for spray painting with an air spray gun so an air pressure regulator is used.
An air pressure regulator placed between the air compressor and the spray gun can be used to adjust the air pressure to the spray gun below the output pressure of the air compressor so an optimal spray paint atomisation can be achieved.
Basically most commonly found air pressure regulators work by way of a valve where the moving part of the valve is pressed against the valve seat with a compression spring and the load on this compression spring can be increased or decreased by way of a threaded adjustment pressing against the other end of the compression spring.
The air pressure regulator restricts the flow of air from the air compressor to the spray gun maintaining the air pressure that the pressure regulator has been adjusted to. It is widely accepted as good practice to begin with the air pressure regulator set deliberately lower than required so under atomisation of the paint while test spraying is occurring. Then increase the pressure at the air regulator until an optimal spray paint atomisation level is occurring while test spraying, this way you are sure that minimum possible air pressure is being used while actually spray painting.
Regulating air pressure to any spray gun to the lowest possible pressure while still maintaining high quality quality paint atomisation will usually ensure that the maximum paint transfer efficiency the spray gun is capable of delivering is realised.
Trying to use an air spray gun without a quality air pressure regulator between the air compressor and spray gun will result in over atomisation and high levels of over spray with poor transfer efficiency. Attempting to regulate the air pressure using the built-in regulator on most air spray guns, if present, is usually difficult and not as effective as using a quality remote air pressure regulator before the spray gun.
Spray gun mounted compressed air regulators are available to attach the the inlet of a spray gun to fine tune inlet air pressure without the need to return to a wall or compressor mounted regulator. A spray gun regulator should not normally be the only compressed air regulator between the spray gun and the air compressor.