are being made by spray gun 20 as by the spray gun trigger 24 being actuated therefore differential pressure switch 47 detects the condition when pressurized air is being delivered through spray gun 20 after the spray gun trigger has been actuated the remaining switch in the series string is pressure switch 14 which is in the normally open position the pressure switch 14 contacts are normally open but become closed whenever the pressure in manifold 45 exceeds a predetermined value in the preferred embodiment this predetermined value is approximately 30 psi therefore under operating conditions where the manifold 45 pressure is higher than 30 psi switch 14 will be in the closed position and the differential pressure across manifold 45 will control the operation of the system in operation the invention permits the system to safely operate under a variety of conditions the 34 prime 34 condition has previously been described but several conditions of operation are also permitted under the 34 spray 34 position of switch 602 if switch 602 is switched to the 34 spray 34 position the material control valve 30 will not permit pressurized liquid to be delivered to spray gun 20 until the spray gun trigger is actuated once the spray gun trigger is actuated the pressurized material is delivered to spray gun 20 until the trigger is deactivated or until certain pressure conditions deviate outside of predetermined limits if the spray gun trigger is deactivated the differential pressure switch immediately.