682 Section 5 Refinishing Copyright Goodheart-Willcox Co., Inc. Balancing the Spray Gun Changing one of the spray gun adjustments will affect the other two. For example, increasing the fan size will decrease the air pressure and spread the paint over a larger area, making a drier spray. Pressure is force divided by area. When one adjustment is made, slight changes may be needed in the other adjustments to achieve the proper balance. For example, if air pressure is turned up to the recommended value after the fan and material are set, the size of the fan may increase. Conversely, if the air pressure is turned down after the fan and mate- rial are set, the fan may decrease in size. In either case, tune the fan and air pressure knobs to get the right size fan at the right air pressure. Also, check the material setting after making these adjustments. Figure 28-9 summarizes the relationship of these adjustments. Pattern Test As outlined in the previous sections, the spray gun must be set up correctly if it is to apply paint as effi- ciently as possible. A pattern test is performed to fine tune spray gun adjustments. Mask off a 36″ × 36″ area on the side of the spray booth with masking paper. Load the spray gun with paint and set up the spray gun by adjusting fan, material, and air pressure. Hold the spray gun the same distance from the masking paper as the height of the fan. If the spray gun produces a 9″ tall oval fan, hold the gun so the air cap is 9″ away from the masking paper. To test a gun set to a 7″ tall fan, hold the gun 7″ away from the masking paper. In one smooth motion, pull the trigger all the way back and immediately release it. Look at the paint pattern. The pattern should be a narrow (1 1/4″ wide) oval with the pattern thinning at the top and bottom of the oval. The thinning pattern is due to the smaller number and size of atomized paint droplets in these areas. If there is a run in the middle of the pattern, turn the fluid adjustment in to decrease the amount of paint. See Figure 28-10. Hold the spray gun at the proper distance and try again. When the runs have been elimi- nated, turn your attention to the top and bottom of the pattern. Adjust the fluid control knob to make the drop- lets at the top and bottom of the pattern as small as possible without making the pattern too faint. Goodheart-Willcox Publisher Figure 28-10. This pattern test shows a run in the center caused by too much paint. Reduce the amount of material. Goodheart-Willcox Publisher Figure 28-9. Changing one of the spray gun adjustments will affect the other two. This table summarizes how adjustments influence each other and how they affect paint. Spray Gun Adjustment – Cause and Effect Adjustment at Gun Effect on Spray Gun Effect on Paint Increase air pressure Increase fan size Decrease material Drier spray Decrease air pressure Decrease fan size Increase material Wetter spray Increase fan size Decrease air pressure Decrease material Drier spray Decrease fan size Increase air pressure Increase material Wetter spray Increase material No effect on fan size No effect on air pressure Wetter spray Decrease material No effect on fan size No effect on air pressure Drier spray DeVilbiss Figure 28-8. This device measures and displays the discharge air pressure.