Spray Gun Operation and Maintenance
What the exam expects
You must understand how a gun's adjustments interact and how to diagnose spray patterns. Know the three primary controls: fluid (needle travel, controls volume), fan/pattern (air to the horns, controls width), and air (inlet pressure at the gun). Understand HVLP versus conventional versus compliant guns in terms of transfer efficiency and inlet pressure.
Core concepts
- Spray pattern diagnosis: a heavy top or bottom indicates a partially clogged horn or a misaligned air cap; a heavy center indicates too little fan air or fluid too thick; spitting indicates a loose or dirty fluid tip/needle or dried material.
- Distance and speed: typically 6–8 inches for many guns, with 50% overlap and a consistent wrist motion. Too far causes dry spray; too close causes runs.
- Viscosity and reduction affect atomization as much as gun settings.
- Cleaning: disassemble, soak solvent-safe parts, clean the air cap holes with a soft tool - never a wire or drill bit that enlarges the orifice.
- Lubrication: use appropriate gun lube on moving parts; never petroleum jelly in air passages.
Traps
Questions often blame the gun for a defect caused by wrong reduction or temperature. Another favorite: "increase fluid to fix a heavy center pattern" - the real fix is more fan air or thinner material. Also watch for answers that recommend probing air cap holes with a hard tool.
Worked example
A gun sprays a pattern heavy at the top. The technician should inspect the top horn's air hole for blockage, check air cap alignment, and verify the cap is seated. Increasing fluid pressure would only worsen the imbalance.
Rule of thumb: adjust in order - air pressure, then fan, then fluid - and test on cardboard before the panel.