When applying paint with a hand held electrostatic paint spray gun, it is desirable to electrically ground the spray gun body to protect the operator from risk of electrical shock. It also is desirable to electrically ground the paint supply to prevent current from flowing from the paint charging electrode at the spray gun front through the paint column to electrically charge the entire paint supply. If the paint supply is isolated from ground and becomes charged, the operator is at risk of receiving a shock when the paint source is replenished or otherwise serviced. For electrically non-conductive paints, the paint column supplied to the gun typically flows through a supply hose to a connector attached to the base of a spray gun handle. A short length of hose is attached between this connector and either the nozzle or the barrel of the spray gun. Although the paint supply hose may be attached directly to the barrel or to the nozzle, supporting the paint supply hose from the handle provides better balance than if the supply hose is attached only to the nozzle or to the adjacent end of the barrel. By attaching the connector to the spray gun handle, the connector is grounded along with the handle. Thus, the paint column flowing to the gun is grounded both at the gun and at the supply. Not only is the operator protected from electrical shock when the entire paint column from the source to the gun is grounded, but the risk of a rupture, a cut or a pin hole break in the paint supply hose allowing a spark from the paint column to ground is eliminated since there is no charge on the paint column. If the paint column were charged and a spark should occur, there is a risk that flammable paint solvents could ignite or explode.
Problems occur when attempting to apply metallic paints with a hand held electrostatic spray gun. The problems result from the short electrical path between the high voltage paint charging electrode at the spray gun nozzle and the normal ground connection to the gun handle. For metallic paints, the high voltage gradient over the short path may cause the metallic particles in the paint to align. This can sufficiently short circuit the high voltage to ground to prevent adequate charging of the atomized paint.
It has been recognized in the prior art that electrically conductive paints can be applied with an electrostatic applicator and with the remote paint source grounded by providing a sufficiently long paint column between the ground point and the paint charging electrode at the spray gun. In U.S. Pat. No. 2,784,350, for example, a length of non-conductive fluid supply hose is formed into a helical coil by wrapping around an insulated support. The coil increases the length of the electrical path from the paint charging electrode to the grounded source. In U.S. Pat. No. 4,139,155, a similar coil is mounted in a cartridge which is attached to the spray gun between the spray gun barrel and a grounded paint supply connector at the lower end of the spray gun handle. This arrangement is effective for lengthening the electrical path between the high voltage paint charging electrode and ground while maintaining the paint column between the gun handle and the supply at ground potential. Thus, the tendency of the metallic flakes to align and to short out the high voltage is reduced and the operator is still protected against the risk of electrical shock in the event that the paint supply hose should break or develop a leak. However, the paint cartridge significantly increases the weight at the barrel of the spray gun, since not only is the weight of the cartridge located between the gun handle and the end of the barrel, but also the weight of the column of paint in the cartridge is present in the cartridge. The added weight to the end of the spray gun adversely affects the balance of the gun and also increases operator fatigue.