678 Section 5 Refinishing Copyright Goodheart-Willcox Co., Inc. Introduction This chapter explains how to use a spray gun. A spray gun uses air pressure to break liquid paint into a mist of tiny droplets. This process is known as atomization. The mist, or fan, is then sprayed onto the surface being painted. The painter is responsible for the setup and operation of the spray gun. The opera- tion of the spray gun is known as spray technique. Spray technique requires the balancing of several variables. One of the variables is speed. The spray gun must be moved at the proper speed across the panel being painted. Another variable is distance. The spray gun must be held the proper distance from the panel. Successful spray painting balances these two key vari- ables, as well as others, to produce a smooth paint job. Paint Characteristics Liquid paint is atomized by a spray gun. The different types of paints used in the typical colli- sion repair shop, such as primers, basecoats, and clearcoats, will have different viscosities. These paints must be properly reduced to acceptable viscosity levels for good atomization. Successful atomization requires properly reduced paint and the correct diam- eter fluid tip. Spray gun fluid tip selection is based on the viscosity of the paint, as explained in Chapter 25. Different types of paint may have different drying char- acteristics. One of the most important drying characteris- tics is flow out. Flow out is the tendency of the wet paint to level and smooth out after spraying. On a vertical surface, such as the door of a vehicle, gravity pulls the paint down, causing the paint surface to flow out. However, if too much paint is sprayed on a vertical surface, the downward force of gravity on the extra paint forms a run. See Figure 28-1. When painting a vertical surface, the technician must pay attention to areas where runs are likely. Places where the body flares and areas where the adjacent panels are perpendicular to each other are danger areas where the potential for runs is high. The technician’s spray technique can also cause runs. Moving too slow, spraying too close, excessive overlap, and improper triggering are the most common causes. Runs are a problem on vertical surfaces because of the downward force of gravity. On flat, hori- zontal surfaces gravity does not pull down on the paint, so runs are usually not a problem. On a horizontal surface, paint may look rough if the painter does not apply a wet coat. A wet coat contains more paint and stays liquid long enough to level out. Because gravity does not level the wet paint on a hori- zontal surface, the paint must stay wet long enough for internal flow out. Internal flow out means the wet paint will continue to level and smooth out until it dries. The wetter the paint, the longer the leveling time. On a vertical surface, apply paint slightly drier to prevent runs. On a horizontal surface, apply the paint slightly wetter for improved internal flow out. Adjustments There are three possible adjustments on spray guns: spray fan size and shape, material flow, and air pressure. See Figure 28-2. Additionally, pressure pot pressure can be adjusted on a pressure-feed spray gun. These adjustments are set to deliver the right amount of paint in a spray fan of the correct size and Goodheart-Willcox Publisher Figure 28-1. Too much paint was applied to this panel. A run was caused by flow out of excess paint. Runs caused by too much paint Goodheart-Willcox Publisher Figure 28-2. The fan knob, fluid control knob, and air pressure control knob are turned in or out to adjust this gravity feed spray gun. Fan knob Fluid control knob Air pressure control knob