679 Chapter 28 Spray Technique Copyright Goodheart-Willcox Co., Inc. shape. The size of the area to be painted and the type of the paint determine what these adjustments will be. Verify that the air compressor can supply the correct volume of air at the specified air pressure. The fan knob, also called the pattern control knob, regulates the amount of air that flows through the air horns. It controls the shape and size of the spray pattern. Turning the fluid control knob adjusts amount of paint or material that is introduced into the airstream. As the fluid control knob is turned out (coun- terclockwise), the trigger can move the needle further and further out from the hole in the fluid tip. Set the fluid control knob to fill the fan with paint. A small fan requires less paint a large fan requires more paint. To set the fluid adjustment knob, turn the knob all the way in. Hang a piece of masking paper on the spray booth wall, hold the gun 6″–8″ away and trigger the spray gun for a second. Almost nothing should come out of the nozzle. Turn the fluid control knob out one full turn and repeat the test procedure. Keep turning the fluid control knob out, a half turn at a time, until you get a full pattern. When set below the maximum adjustment, the setting of the fluid adjustment knob will allow less than the maximum amount of paint to mix with the air. This is important for atomization. Orange peel, an undesirable textured, dimpled surface, results when air pressure cannot break the paint into small droplets. If too much paint and not enough air are discharged from the spray gun, atomization is poor and orange peel is the result. If orange peel is present, the fluid adjustment knob can be turned in to decrease the amount of paint. This will change the ratio of paint to air, improving atomization. The air pressure control knob is used to adjust air pressure. Adjusting air pressure is as important as setting the fluid controls. Insufficient air pressure leads to poor atomization, which causes orange peel. Too much air pressure blasts paint out of the spray gun, causing excessive overspray and a dry, rough finish. Also, the size of the fan is determined by air pressure. Spray Gun Setup A properly adjusted spray gun will apply a smooth coat of paint. As previously mentioned, there are three adjustments on a spray gun: fan size, material flow, and air pressure. Spray gun setup is a balance of these three adjustments, with a separate knob controlling each one. The paint manufacturer or spray gun manu- facturer will specify the air pressure setting. The other two adjustments, fan size and material flow, must be determined and set by the technician. An improperly NOTE adjusted spray gun will still apply paint, but the results will be less than satisfactory. Also, the spray gun should be clean before attempting setup. There are many ways to set up a spray gun, but it is important to remember that all of the adjustments are interrelated. Changing one will have an effect on the other two. Methods for setting up both conventional and HVLP spray guns are discussed in the following sections. Keep in mind that the methods presented are not the only ways to set up a spray gun. Setting up a Conventional Spray Gun Fill the cup with paint and connect the spray gun to an air hose supplying approximately 40 psi of air. Set the fan first. Set the shape of the fan based on the size of the area to be sprayed. See Figure 28-3. When the fan knob is turned all the way out, the maximum amount of air hits the spray mist, making a narrow (7″–12″ tall), oval-shaped fan. As the knob is turned in, the amount of air delivered Goodheart-Willcox Publisher Figure 28-3. Spray a small area with a small fan spray a large area with a large fan. Small area Large area Small fan Large fan