A commonly used type of hand held paint spray guns has a paint cup attached to extend below a barrel portion of the gun. Compressed air is supplied to the gun through a hose attached to the butt of a handle. The compressed air is supplied under the control of a trigger actuated valve to a nozzle for atomizing paint and often for shaping the pattern of the atomized paint. In one type of spray gun, when a trigger actuated fluid valve is opened, the flow of compressed air aspirates or sucks paint from a paint cup into the nozzle where the compressed air carries the paint through an orifice and atomizes the paint. For this type of spray gun to function, the paint cup must be vented to atmosphere. Otherwise, as paint is consumed, a vacuum will build up in the paint cup and paint feed will cease. In another type of spray gun, a portion of the compressed air is supplied to the interior of the paint cup for pressurizing the paint cup. The pressure causes the paint to flow from the cup to the nozzle, where it is discharged with air through an orifice and atomized when a trigger actuated fluid valve is opened. Pressure assisted paint feed is used with paints which are too viscous for suction feed or for applications where fast coverage is more important than the quality of the finish.
In the past, most spray guns of the suction feed type were not designed to operate satisfactorily in a pressurized feed mode since they were not constructed to supply compressed air to the paint cup. If a separate compressed air line is connected from the compressed air source to the paint cup, paint can flow into the atomization air and pattern shaping air passages if the paint pressure exceeds the air pressure in the spray gun.