Materials, Consumables and Surface Preparation
This domain covers abrasives, fillers, primers, masks, cleaners and the preparation steps that decide whether paint sticks and lasts. The exam tests sequence as much as products: what you use, in what order, and why a shortcut causes a defect.
Core rules
- Identify the substrate first: steel, aluminium, galvanised steel, plastic or factory paint. Each needs a different preparation and primer.
- Wash and degrease before sanding. Sanding drives contamination into the surface; a water-based degreaser followed by a clean water rinse and drying is usual, with a solvent wipe for some substrates.
- Use grit in steps. Coarse grades such as P60-P80 strip paint and shape filler; P120-P180 smooth filler; P240-P320 prepare primer; finer grades prepare blend areas. Jumping grades leaves scratches that swell through the topcoat.
- Mix body filler according to the maker's ratio, usually about 2% hardener by weight for polyester filler. Too little hardener gives a soft, sticky film; too much gives a brittle, porous repair. Work within pot life and in the stated temperature range.
- Feather the repair edge so the repair is invisible under a straight edge. Apply etch primer to bare steel or aluminium where specified, epoxy primer for corrosion protection, and high-build primer to fill sanding scratches.
- Use a tack cloth immediately before painting and keep masking edges clean. Mask plastics and rubbers properly and remove masking before the paint is fully hard, following the product instructions.
- Store fillers, primers and solvents closed, upright and within temperature limits; check shelf life and never mix brands without approval.
Worked example
A filler repair stays tacky after 30 minutes. The likely causes are too little hardener, cold workshop temperature, contaminated mixing board or an expired product. The correction is to remove the soft filler, clean back to sound substrate, mix a fresh batch at the correct ratio on a clean board, and allow the specified cure time. Adding more hardener to the old batch is not a fix.
Traps that cost marks
Sanding through a feathered edge, using the wrong primer on aluminium, skipping degreasing, mixing filler by eye, and painting over dust.