Spray Painting
Spray Painting
This domain tests atomisation, gun setup, gun handling and defect avoidance in spray application. Expect questions on air pressure, fluid tip size, fan pattern and distance.
Principles and setup
- A spray gun atomises paint by mixing fluid with air at the cap. Fluid tip, needle and air cap must match the product viscosity.
- Feed systems are suction, gravity and pressure. Gravity guns work at lower pressure and suit small jobs.
- HVLP uses high volume at low pressure, often around 10 psi at the cap, and gives high transfer efficiency.
- Airless spraying uses hydraulic pressure, not air, to atomise; it suits large areas and high-build coatings.
- Electrostatic spraying charges the paint so it wraps the workpiece; it needs an earthed workpiece and a safe gun-to-target distance.
Handling rules
- Hold the gun 150–200 mm from the surface and keep it perpendicular.
- Use 50% overlap between passes and trigger at the start and end of each pass.
- Adjust fluid and air so the test pattern is even, with no heavy centre or split ends.
- Keep the gun moving at a steady speed; stopping causes runs.
Worked example
A gravity gun produces a pattern with heavy paint in the centre and split ends. The likely cause is too much fluid or too little atomising air. The fix is to open the air control, reduce fluid, and clean the air cap horns. The reasoning is that the horns shape the fan, and blocked holes distort it.
Traps
- Holding the trigger while changing direction, which doubles the film at the ends.
- Spraying too far away and causing dry spray and overspray.
- Using a fluid tip that is too small and getting a spitting, uneven pattern.
- Arcing an electrostatic gun toward earthed objects.