Surface Preparation, Finishing and Adhesives
This domain tests the sequence from bare timber to finished surface, plus correct adhesive selection. Most lost marks come from sequence errors - a skipped grit or the wrong glue for the environment.
Preparation
- Grit sequence 80 → 120 → 180 → 220/240, never skipping more than one step; each grit removes the scratches of the previous one.
- Sand with the grain; use a block on flat faces; avoid rounding arrises.
- Raise the grain with a damp cloth, allow to dry, then denib with 240 grit before the first coat.
- Scrape rather than sand highly figured timber to avoid tear-out. Remove all pencil and glue marks, since finish will not hide them.
Finishes
- Open-pored timbers such as oak, ash and mahogany need a paste wood filler for a level film.
- Products: nitrocellulose lacquer (fast, low resistance), acid-catalysed lacquer (durable, kitchens), two-pack polyurethane, water-based acrylic (low VOC, raises grain), hard wax oil, shellac and French polish.
- Film build: 3 coats at 80–120 g/m2 wet, denibbed between coats with 320–400 grit and dusted off.
- Spraying: HVLP at 2–3 bar, viscosity matched to the gun, 50% overlap, consistent distance.
- Defects: blushing (moisture in the line), orange peel (viscosity or cold), runs, sand-through, fish-eyes (silicone contamination).
Adhesives
- PVA to EN 204: D2 interior, D3 moisture-resistant, D4 exterior/wet. Polyurethane: waterproof, foaming, gap-filling. Epoxy: structural, gap-filling, long open time. Contact adhesive: both faces, tack-dried, not structural. Urea-formaldehyde: long assembly time.
- Wood at 8–12% MC; workshop at least 10 °C for PVA (5 °C absolute minimum); clamp pressure about 0.5–1.5 N/mm2; clamp 30–60 minutes, full strength at 24 hours.
- Coverage roughly 150–200 g/m2 of glue line. Squeeze-out should be a thin bead, not a flood.
Worked example
A cabinet presents 4.2 m2 and needs three coats at 100 g/m2 wet. Wet finish on the surface = 4.2 × 3 × 100 = 1260 g. With HVLP transfer efficiency of 60%, product consumed = 1260 / 0.6 = 2100 g, about 2.1 kg. Order 2.5 kg to cover loss in the gun and pot.
Traps that cost marks
Skipping grits, which only shows after the first coat of lacquer; sanding across the grain; too much glue causing slip and squeeze-out; using PVA below 5 °C; contact adhesive on a structural joint; failing to seal end grain, which absorbs unevenly.
Justify adhesive choice by environment and loading, not by habit.