681 Chapter 28 Spray Technique Copyright Goodheart-Willcox Co., Inc. Setting up an HVLP Spray Gun An HVLP spray gun is set up in much the same way as a conventional spray gun. In fact, the first two adjustments are the same. The air pressure setting, however, is different. There are two ways to set air pressure on an HVLP spray gun. The conventional way is to first check the maximum inlet air pressure. HVLP spray guns have a maximum inlet air pressure setting engraved into the body of the spray gun. This pressure is determined under ideal conditions, maximum diameter air hose, low restriction air fittings, and properly sized air compressor. See Figure 28-7. If the indicated maximum inlet air pres- sure is supplied to an HVLP spray gun, the maximum discharge air pressure for the HVLP spray gun (10 psi) will be discharged. This is a less precise method of setting pressure because pressure below 10 psi can only be estimated. If more than the specified maximum inlet pressure is supplied to an HVLP spray gun, the discharge pressure will be greater than 10 psi. At a discharge pressure greater than 10 psi, the HVLP spray gun will no longer function as intended. The other method of setting air pressure on an HVLP spray gun involves the use of a special gauge assembly to measure the discharge air pressure. See Figure 28-8. The gauge assembly is installed on the spray gun in place of the normal air cap. The discharge air pressure is set to 10 psi by adjusting the air regulator while air is streaming through the spray gun. Once the desired air pressure is set, the special gauge assembly is removed and the normal air cap is installed. After initial adjustment, the technician can reduce air pres- sure at the regulator if needed. However, the air pres- sure should not be increased because this will cause more than 10 psi to be discharged. This special gauge assembly allows the spray gun to be set accurately at pressures below 10 psi. Regulator Goodheart-Willcox Publisher Figure 28-5. Set the air pressure at the regulator on the spray booth wall with air flowing through the spray gun. 29 psi Goodheart-Willcox Publisher Figure 28-7. The air pressure indicated, 29 psi, at the inlet will make an air cap discharge pressure of 10 psi. Goodheart-Willcox Publisher Figure 28-6. The air gauge is attached to the spray gun. The pressure at the inlet of the spray gun is indicated on the gauge. Adjust the spray gun’s air pressure with a regulator.