680 Section 5 Refinishing Copyright Goodheart-Willcox Co., Inc. to the air horns decreases, changing the shape of the fan from an oval shape to a 2″-diameter round pattern. When painting a large area, such as a hood, the fan adjustment should be set to make the tall, oval-shaped fan. This will cover the surface with the minimum number of passes. When painting a small area, such as the inside of a doorjamb, the spray gun should be set to a small circle. This will paint the small area with the minimum amount of overspray. After setting the fan, the fluid control knob should be adjusted to the appropriate setting. The fluid control knob is used to adjust the distance the needle moves away from the hole in the fluid tip when the trigger is pulled. Trigger movement, beyond the stop for the air valve, is adjusted by the fluid control knob. Turning the fluid control knob all the way in (clockwise) prevents the trigger from moving the needle back and exposing the hole in the fluid tip. Turning the knob out (counterclockwise) allows the trigger to move the needle out of the fluid tip hole and release paint into the compressed airstream. When the trigger moves back far enough to touch the body of the spray gun, material flow is at the maximum. Some technicians turn the fluid control knob all the way in and then count the number of turns out. This gives them an idea of what the material setting is. If the fan is set at maximum, then the material flow should also be set at maximum. The material should fill the fan. If the fan is set at less than maximum, the material flow should be decreased accordingly. After adjusting the fan and material settings, the air pressure must be set. As previously stated, air pres- sure recommendations are provided by the paint or spray gun manufacturer. See Figure 28-4. The best way to set up air pressure is not at the gun, but at the regulator. A regulator has a diaphragm that adjusts the air pressure with no loss in air volume, while a valve sets air pressure by restricting air volume. Set the air pressure control knob on the spray gun to maximum and use the regulator and gauge on the spray booth wall to set air pressure. The air pressure should be set with the spray gun trigger pulled all the way back to open the air valve. With air flowing out of the spray gun, the regulator is set to the desired air pressure. The air pressure supplied to the spray gun will be the pressure that is discharged. Figure 28-5 shows a technician setting the air pressure at the regulator. In this example, a short length of hose is used. A hose that is less than 30′ long with an interior diameter of 5/16″ and with one quick-disconnect fitting at the spray gun, helps minimize the air pressure drop in the hose between the regulator and the spray gun. An inline air pressure gauge is used to check the air pressure at the spray gun air inlet. See Figure 28-6. Goodheart-Willcox Publisher Figure 28-4. This chart gives one manufacturer’s air pressure recommendations for various spray gun types and paint products. Spray Gun Types and Recommended Air Pressure Waterborne Basecoat Spray Gun Type SATA JET 5000 B HVLP SATA JET 5000 B RP SATA JET 4000 B HVLP SATA JET 4000 B RP Standard Conditions: 68°–90°F (20°–32°C)/60% Relative Humidity or Less Fluid Tip Size* WSB 1.2W WSB 1.1/1.2 Coverage Coat Pressure 22–26 psi 22–28 psi 24–29 psi 26–32 psi Control Coat Pressure 16–18 psi 16–18 psi 16–18 psi 17 psi Standard to High Temp/High Humidity: 68°F (20°C) and above/60% Relative Humidity or More Fluid Tip Size* WSB — WSB 1.1 Coverage Coat Pressure 22–26 psi — 24–29 psi 26–32 psi Control Coat Pressure 16–18 psi — 16–18 psi 17 psi High Temp/Low Humidity: 90°F (32°C) and above/30% Relative Humidity or Less Fluid Tip Size* WSB/1.3 — 1.3 1.2 Coverage Coat Pressure 26–29 psi — 22–26 psi 26–32 psi Control Coat Pressure 16–18 psi — 16–18 psi 17 psi *WSB fluid tips are specifically designed to provide proper atomization of waterborne basecoats. The letters WSB stand for water solvent borne.