Apply Paint and Coatings by Brush, Roller, and Spray
This domain tests the actual application of coatings by brush, roller and spray, including technique, film build, environmental limits and defect avoidance.
Core rules
- Brush: load about one-third of the bristle length, cut in clean edges, apply in a consistent direction, then lay off lightly with the grain or vertical direction to level marks.
- Roller: choose nap for the profile, wet the roller evenly, apply in a W pattern and back-roll in one direction while the edge is wet. Do not over-roll as the coating sets.
- Spray: hold the gun perpendicular at the manufacturer's distance (commonly 150–200 mm), overlap about 50%, start movement before triggering, and stop before changing direction. Adjust fluid and air pressure to get a uniform pattern. Use a test card.
- Maintain a wet edge. Recoat within the stated window; if the window is exceeded, sand to key the surface before recoating.
- Check substrate temperature, air temperature, humidity and dew point. Do not apply when the surface is below the dew point or outside the data sheet range.
- Confirm wet film thickness with a gauge and calculate dry film from volume solids.
Worked example
A specification requires 50 µm dry film thickness with a coating of 60% volume solids. WFT = DFT ÷ volume solids = 50 ÷ 0.60 = 83 µm. If the gauge reads 55 µm wet, the dry film will be about 33 µm, below specification. The applicator should adjust technique or pass rate, not simply add thinner. The reasoning is that thinner raises WFT without raising solids, so DFT remains low.
Traps
- Over-thinning to make brushing easy, which reduces hiding and durability.
- Holding the spray gun still or arcing it, causing runs and uneven film.
- Recoating too soon (solvent trapping) or too late (poor adhesion).
- Painting in direct sun or on a hot surface that flashes the solvent.