Spraying, Painting and Finishing
This domain tests spray gun setup, application technique, defect prevention and finishing. The exam wants you to explain why a finish runs, dries rough or mismatches, and to choose a controlled, repeatable process.
Core rules
- Set the gun for the product: a smaller fluid tip for base coat and clear coat, a larger tip for primer. Check the manufacturer's air pressure at the cap and use a regulator at the gun.
- Check viscosity with the specified flow cup and reduce only as the product sheet allows. Over-thinning causes runs, poor coverage and weak clear coat.
- Hold the gun about 15-20 cm from the panel, keep it at 90 degrees, and overlap each pass by about 50%. Move at a steady speed from the wrist and shoulder, not by fanning the elbow.
- Apply a tack coat first, allow the specified flash-off, then build coats. Keep booth temperature and airflow stable. Do not spray a cold panel or in a dusty area.
- For base coat and clear coat, follow the product system. Clear coat needs the correct wet film build; too little gives dullness, too much gives runs and solvent pop.
- Match colour using the variant card and a spray-out card, and blend into adjacent panels where the system allows.
- Finish defects: runs come from too close, too slow or too much material; dry spray from too far, too fast or too much air; orange peel from viscosity, pressure or gun distance. Remove nibs and dust with fine abrasive, then compound and polish.
Worked example
A clear coat shows runs on a vertical door. The technician wants to add more reducer. The correct reasoning is that reducer will make the problem worse. The causes are likely too heavy a coat, too slow a pass, too close a distance or too much fluid. The fix is to stop, let the coat flash, adjust the gun to the specified setting, apply thinner coats with correct overlap, and polish the cured runs if needed.
Traps that cost marks
Judging viscosity by eye, skipping the tack coat, spraying too close to edges, mixing products from different systems, and polishing before the clear is fully cured.