Adhesives, abrasives, finishes and wood preservation
What the exam tests
Matching adhesive, abrasive and finish to the substrate and service conditions, and applying preservation correctly.
Core rules
- PVA: interior, D3 for humid interior, D4 for exterior non-structural; not for continuous loading or structural use unless specified. PU: exterior and some structural, needs moisture and clamping. Epoxy: gap-filling, strong, expensive. Contact adhesive: instant, poor creep resistance.
- Adhesive bonds need clean, dry, close-fitting surfaces, correct spread, temperature and clamp pressure. Over-clamping starves the joint.
- Abrasives: work through grits (e.g. 80, 120, 180) and remove dust between grits. Stearated paper resists clogging on finishes; belts and discs need correct speed.
- Finishes: oil penetrates and is easy to repair; wax is not a durable exterior finish; shellac is alcohol-soluble; lacquer and water-based finishes need compatible sealers; PU needs adequate ventilation and cure.
- Preservation: pressure treatment drives preservative deep; brush-on is surface only. Cut ends and drill holes in treated timber must be re-treated. Use biocides only to label and SDS.
Worked example
Exterior oak gate: choose a PU or epoxy adhesive, not interior PVA. Seal end grain, use stainless fixings, and finish with a breathable oil or exterior stain. The reasoning: exterior moisture cycles demand a water-resistant glue and a finish that moves with the wood; film-forming finishes may crack.
Traps
Sanding through grits and polishing the surface so glue cannot key; using D3 PVA outdoors; over-clamping; applying finish over wax or dust; treating cut ends as if they were factory-treated; mixing preservative and finish without checking compatibility.