Surface Preparation and Finishing
This domain tests how you prepare a surface and build a finish that looks even and holds up - sanding sequence, stain and coating selection, spray technique, and the safety and environmental rules around a spray booth. Questions often present a finish defect and ask for the cause.
Surface preparation
- Sand in sequence and do not jump more than one grit: 80, then 120, then 150, then 180 to 220 for the final coat. Each grit removes the scratches of the previous one.
- Sand with the grain; use a sanding block on flat work; sand sealer lightly with 320 or finer.
- Raise the grain with a damp cloth before staining when using waterborne products, then sand back lightly.
- Remove dust with a tack cloth or compressed air and a clean environment; dust nibs show up under gloss.
Stains and coatings
- Pigment stains sit on the surface and highlight figure; dyes penetrate and give more even colour.
- Blotch-prone species (maple, cherry, pine, birch) need a washcoat or conditioner before pigment stain.
- Nitrocellulose lacquer, pre-catalyzed and post-catalyzed lacquer, conversion varnish, polyurethane, shellac, waterborne acrylic and oil each have different build, cure and recoat windows - respect the recoat window or the coat will lift.
- Sealers and toners even out colour; do not over-sand sealer or you cut through to bare wood.
Spray technique
- HVLP guns: fluid and air adjusted so the pattern is even, gun about 150 to 200 mm from the surface, 50 percent overlap per pass, wrist moving parallel to the surface.
- Thin only per the product data sheet; measure viscosity with a Zahn cup when specified.
- Build thin coats; heavy coats sag, blush or trap solvent.
Safety and environment
- Spray in a booth with airflow away from the operator and explosion-proof electrics; bond and ground containers and parts.
- Wear the right respirator for the product - isocyanate and solvent products need more than a dust mask.
- Keep the booth filters clean and dispose of solvent waste and rags per regulations.
Worked example
A maple tabletop stains blotchy. Reasoning: maple has dense and porous zones that absorb pigment unevenly. Correct sequence: sand to 180, apply a thin washcoat of thinned sealer or a commercial conditioner, let it dry, then apply a dye rather than a heavy pigment stain. Wiping the stain evenly and quickly reduces lap marks. Sanding to a finer grit would only burnish the surface and make absorption worse.
Traps that cost marks
- Skipping grits, leaving coarse scratches that show under gloss.
- Sanding through veneer while trying to remove a stain.
- Applying a second coat outside the recoat window without abrading.
- Spraying without grounding the part or the container.