697 Chapter 28 Spray Technique Copyright Goodheart-Willcox Co., Inc. Rinse the air cap, filter, and cup with aqueous cleaning solution in the machine. Pull the trigger on the spray gun and hold the spray gun over the machine. Run aqueous cleaning solution through the spray gun. Blow the air cap, cup, and spray gun dry and wipe with clean towels. If a spray gun cleaning machine is not available, fill the cup about one fourth full with aqueous cleaning solution, trigger the spray gun into the aqueous waste barrel, and let all the solution move through the spray gun. Remove the air cap and rinse it with water. Reinstall the air cap. Attach an air line to the spray gun and fill the cup about one fourth full of water and spray the water through the spray gun. Blow the spray gun dry and wipe with paper towels. If the spray gun has a disposable cup, use a squeeze bottle to run water through the spray gun as the gun is trig- gered over the aqueous waste barrel. Plastic gaskets inside the body of the spray gun prevent paint from leaking around the needle. These plastic gaskets swab the needle as the needle moves in and out of the gasket. The gaskets should be treated with spray gun lubricant for proper function. There are also gaskets in the air valve. These gaskets prevent air leaks. As with needle gaskets, these gaskets should be kept lubricated. Summary • Speed and distance are the two key variables of spray technique. • Flow out is the tendency of wet paint to level and smooth out after spraying. The wetter the paint, the longer the leveling time. • There are three possible adjustments on spray guns: fan size and shape, material flow, and air pressure. • The pattern control knob regulates the amount of air that flows through the air horns. It controls size and shape of the spray pattern. • The fluid control knob controls the amount of paint that is introduced into the airstream. • The air pressure control knob is used to adjust air pressure. • Changing one of the spray gun adjustments will affect the other two. • A pattern test is performed to fine-tune spray gun adjustments. • A flood test is a check of paint distribution within the spray pattern. NOTE • While spraying, the paint technician must watch the paint as it is applied and keep an eye on the path ahead. • Once the first stroke (one pass of the spray gun at a specific distance and speed) is made, all subsequent strokes should match the initial stroke. • Proper spray technique is easier to maintain if the painter is positioned near one end of the panel. • For consistent paint coverage, the fan must strike the surface uniformly. To keep the fan uniform, the technician must tilt the spray gun to follow the panel contour. • Travel speed should be about 1 1/2′ per second for HVLP guns and 1′ per second for conventional spray guns. • When painting, each stroke should overlap the previous stroke by 50%. • A medium wet coat is the most common. • Spray guns must be cleaned after every use. Activities 1. Create a one-page poster on spray gun setup. Use drawings to illustrate each step. Include adjustments to set pattern size, material flow, and air pressure. 2. Triggering a spray gun is a concept that is often difficult for a beginner to learn. Using an actual spray gun your instructor provides to you, demonstrate proper triggering to the class. Explain how to trigger when painting a long panel in sections. Review Questions Answer the following questions using the information provided in the chapter. 1. Paint leveling after spraying is called _____. 2. Decrease in air pressure will cause a drier spray. True or False? 3. Material, fan, air pressure is the recommended sequence when setting up a spray gun. True or False? 4. The _____ moves out of the air cap when the spray gun trigger is pulled back. 5. If an HVLP spray gun is supplied with 20 psi of air at the inlet, it will discharge 20 psi from the air cap. True or False? 6. Decreased travel speed will apply a wetter coat of paint. True or False?