Spray Painting: Materials, Mixing, Spray Gun Setup and Application
Products and mixing
Primers, sealers, basecoats and clearcoats come as 1K (single-pack, air-dry) or 2K (two-pack, catalysed). Mix by volume using the manufacturer's ratio and a measuring stick; never guess. Choose reducer or activator by shop temperature: too fast in heat gives dry spray, too slow gives runs and solvent pop. Respect pot life; mixed 2K clearcoat is unusable after its pot life.
Gun setup
- Fluid tip: about 1.2–1.4 mm for basecoat and clearcoat, 1.6–1.8 mm for primer surfacer.
- Set air pressure with the gun triggered and the fluid needle open; HVLP runs at low cap pressure, roughly 0.7–1.5 bar.
- Test the pattern on paper: a long oval with even distribution, no split or heavy centre.
- Adjust fluid and fan before starting, not in the middle of a panel.
Application
Hold the gun 15–20 cm from the surface, perpendicular, and move at a steady speed from the wrist or shoulder. Overlap each pass by about 50%. Apply a light tack coat first, flash off, then full coats. For metallics, use a drop coat at reduced pressure or increased distance to orient the flake.
Worked example
A clearcoat is sprayed with a 1.8 mm tip at high pressure and sags appear on the door. The tip is passing too much material. Fit a 1.3 mm tip, set pressure correctly, and apply two medium coats with proper flash-off. Reasoning: film build is controlled by fluid tip, pressure and speed together.
Traps
- Guessing mixing ratios or using the wrong reducer.
- Spraying a cold panel or in a cold booth.
- Arcing the gun instead of keeping it parallel.
- No tack coat, causing pinholing.
- Painting over wet primer.