683 Chapter 28 Spray Technique Copyright Goodheart-Willcox Co., Inc. Common problems that can be seen in the spray pattern are shown in Figure 28-11. These include: • Sickle pattern—A clogged or partially blocked air horn will cause a sickle- or comma-shaped pattern. Clean out the air horn hole with a wooden toothpick. • Hourglass pattern—In an hourglass pattern, the center is dry, and the top and bottom are wet. Likely causes are too much air pressure or not enough material. The solution is to decrease the air pressure or increase material flow. • Wide pattern—There is not enough air pressure or too much material. Increase the air pressure and/or reduce the material. • Split pattern—Blockage in the air cap or fluid tip. Clean the air cap by removing and soaking it in thinner. Clean air cap holes with a wooden toothpick. Wipe the fluid tip with a rag soaked in thinner to remove dried paint. • Spitting—Spitting means that the paint discharge is not consistent. The spray gun alternately discharges paint and then air. This is usually caused by a loose fluid tip. Tightening the fluid tip will correct the problem. • Bubbling (not shown in Figure 28-11)—Bubbling, heard or felt at the spray gun, is caused by air inside the spray gun moving through paint passages. A loose fluid tip can cause the problem as well as a shrunken or damaged plastic gasket or O-ring. Flood Test A flood test is a check of paint distribution within the spray pattern. A flood test is made on a sheet of masking paper taped to the spray booth wall. Turn the air cap so the air horns are oriented up-and-down rather than side-to-side. This will make the fan hori- zontal instead of vertical. Position the spray gun the same distance from the masking paper as the length of fan pattern. Hold the spray gun stationary and fully pull the trigger back and hold it for 4 seconds. Four seconds is a long time, and paint will flood the spray pattern and run down the masking paper like a waterfall. If the runs are uniform from left to right, the paint distribu- tion is correct. If, for example, there are heavy runs on one side of the spray pattern and fewer, lighter runs on the other side, then paint distribution is not uniform and the spray gun must be adjusted accordingly. See Figure 28-12. An incorrectly adjusted spray gun cannot apply paint efficiently. No amount of spray gun technique can compensate for an improperly adjusted spray gun. Learn how to correctly set up the spray gun. Goodheart-Willcox Publisher Figure 28-11. A pattern test will show if the spray gun is adjusted correctly or if it needs cleaning. Pattern Test Name Shape Correction Sickle Clean air horn holes Hourglass Decrease air pressure or increase material Wide Increase air pressure or decrease material Split Clean air cap or fluid tip Normal No correction needed, good pattern Goodheart-Willcox Publisher Figure 28-12. A flood test also shows if the spray gun needs cleaning or adjustment. Flood Test Name Shape Correction Good, uniform runs None Heavy on one end Clean air cap or fluid tip Heavy on both ends Decrease air pressure or increase material Heavy center Increase air pressure or decrease material