Wood Finishing Materials and Techniques
Wood finishing involves the application of protective or decorative coatings to a wood substrate. Choosing the correct finish requires an understanding of the chemistry of the coating, the properties of the wood species, and the intended use of the finished object.
1. Stains and Colorants
Stains modify the color of the wood without forming a significant film on the surface. They rely on pigments or dyes suspended in a carrier.
1.1 Dyes vs. Pigments
- Dyes: These are microscopic particles that dissolve completely in the carrier (water, alcohol, or oil). They penetrate deep into the wood fibers and are transparent, making them ideal for highlighting figured grain without obscuring it.
- Pigments: These are larger, opaque particles that stay in suspension. They tend to settle into the larger pores of the wood (like in oak or ash) and provide better UV resistance than dyes but can "muddy" the grain if applied too heavily.
1.2 Chemical Stains
Some woods can be colored through chemical reactions. Fuming with ammonia reacts with the tannins in white oak to produce a deep brown or gray color. Ferrous sulfate (iron buff) reacts with tannins to turn wood silver-gray or black.
Film-forming finishes create a protective barrier on top of the wood.
2.1 Varnish
Varnishes are composed of resins, drying oils, and solvents. They cure through oxidative polymerization—a chemical reaction with atmospheric oxygen.
- Alkyd Varnish: Traditional varnish made from alkyd resins. It provides good durability and a warm, amber tone but is slow-drying.
- Polyurethane: A varnish that incorporates polyurethane resin for superior abrasion and water resistance. It is available in oil-based and water-borne formulations.
2.2 Lacquer
Lacquer is a fast-drying finish that hardens through solvent evaporation.
- Nitrocellulose Lacquer: The traditional standard for furniture. It offers excellent clarity and "burns in" to previous coats, meaning the solvent in a new coat slightly dissolves the previous one to create a seamless bond.
- Pre-catalyzed and Post-catalyzed Lacquers: These incorporate resins that cross-link after application, providing significantly higher durability and chemical resistance than standard nitrocellulose.
2.3 Shellac
A natural resin secreted by the lac bug, dissolved in denatured alcohol. It is a "spirit varnish" that dries very quickly. While not as heat or alcohol-resistant as modern finishes, it is non-toxic, easy to repair, and an excellent sealer for blocking resin or silicone contamination.
3. Curing and Drying
- Drying: The physical process of the solvent evaporating. A finish may be "dry to the touch" but still chemically unstable.
- Curing: The chemical process of polymerization where the finish molecules cross-link to form a hard, durable film. This can take anywhere from a few hours for lacquers to several weeks for oil-based varnishes.
4. Application Techniques
4.1 Surface Preparation
The quality of the finish is dependent on the quality of the sanding. Any scratches left from lower-grit sandpaper will be magnified by the finish. For most hardwoods, sanding up to 180 or 220 grit is sufficient before applying a film-forming finish.
4.2 Grain Filling
Open-pored woods like oak, walnut, and mahogany may require a grain filler to achieve a perfectly smooth, "poured-on" glass finish. Fillers are thick pastes applied to the surface and squeegeed into the pores before the final topcoats are applied.
4.3 Sealing
A sealer coat (often a wash-coat of shellac or a thinned version of the final finish) prevents the wood from absorbing subsequent coats unevenly. It also prevents "blotching" in woods like cherry or pine when staining.
5. Troubleshooting Common Issues
- Blotching: Caused by uneven absorption of stain. Use a pre-stain conditioner or a wash-coat of shellac to equalize the surface.
- Orange Peel: A bumpy surface texture caused by poor leveling. Often due to the finish drying too quickly or improper spray gun settings.
- Fish Eyes: Small circular craters caused by surface contamination, usually silicone from furniture polishes or lubricants.
- Blushing: A milky white haze in lacquer caused by moisture being trapped in the finish during high humidity. Use a lacquer retarder to slow the drying time and allow the moisture to escape.