Vehicle Body, Chassis, and Paint Work
Domain 8 tests chassis and body construction and the sequence of a paint repair, including defect diagnosis.
Chassis and body
A ladder frame (two rails with cross-members) suits load carriers; backbone frames and monocoque (integrated body-shell) construction suit cars. Panels are welded, bolted or bonded, and crumple zones absorb crash energy instead of passing it to occupants.
Paint repair sequence
Wash and degrease; strip to bare metal where needed; remove dents by panel beating or a pulling tool; apply body filler in thin layers; sand 80 → 180 → 320 grit; primer; wet-sand 600–800; mask; base coat in two or three coats; clear coat; bake or air-dry; polish with compound. Hold the spray gun about 15–20 cm away, square to the panel, 50 % overlap.
Defects: orange peel (too thick, wrong thinner, gun too far, low pressure); runs (too much material, gun too close); fish eyes (silicone or oil contamination); blisters (moisture under the paint); sanding scratches showing through.
Worked example
Fresh clear coat shows a fine bumpy skin, with no dirt specks and no runs. Reasoning: the texture is atomised droplets that failed to flow out, so it is orange peel rather than contamination. Cure by wet-sanding with 1200–1500 grit and compounding, then correct thinner ratio, pressure and gun distance on the next panel.
Traps that cost marks
Painting over wax or silicone; skipping primer; using too fine a grit before the filler is shaped; sanding through the clear coat on edges; guessing film thickness instead of using a gauge.