What is the difference between pressure- and siphon-feed paint spray guns?

In a siphon-feed spray gun, the paint canister is open to atmospheric pressure and paint is siphoned as it is sprayed by a partial vacuum created in the spray gun. This method of paint feed is best for feeding light-bodied paints and lacquers and can only be used when spraying with external mix caps.

In a pressure-feed spray gun, the canister or paint tank is pressurized to force paint to feed to the gun. Pressure feed is generally used for spraying heavy-bodied paints, house paints, etc., and can be used with either an internal- or external-mix air cap.