Spray Coating Application: Equipment, Parameters, and Control
Core concepts
Spray coating is a system of linked variables: coating viscosity, gun setup, atomizing and fan air, fluid delivery, spray distance, angle, overlap, and pass speed. Changing any one shifts film build, overspray, and appearance. A candidate must understand the difference between air atomization, airless, air-assisted airless, and plural-component systems, and how each converts fluid into droplets. Know that airless relies on hydraulic pressure through a small orifice, so pressure and tip size jointly set flow; air-assisted airless adds air to refine the fan.
Parameters and their effects
Higher viscosity or lower fluid pressure reduces flow; more atomizing air produces finer droplets but increases overspray and dry spray. Spray distance that is too far causes dry, sandy film and loss of gloss; too close causes runs, sags, and heavy wet edges. Overlap of roughly 50 percent on a consistent pattern, with the gun held perpendicular to the surface, produces uniform build. Wet-film thickness relates to dry-film thickness through volume solids: dry film equals wet film times volume solids fraction.
Traps in questions
Watch for distractors that confuse atomizing air with fan air, or that claim more pressure always improves finish. Questions often invert the cause-effect of dry spray (too much air/distance, not too little). Another trap is confusing tip orifice size with fan width; they are separate designations.
Worked rule of thumb
If a coating has 50 percent volume solids and you need 4 mils dry, target 8 mils wet. If you measure 6 mils wet, expect about 3 mils dry and adjust passes or fluid flow accordingly.