Lacquer has traditionally come from tree resin mixed with a solvent (lacquer thinner). In the mid-1900s, nitrocellulose resin was discovered by the auto industry and added to lacquer. The unique property of adding nitrocellulose resin to lacquer is that each coat dries into the next coat, resulting in a very hard finish that is flexible enough to not crack. The drawback is that it deteriorates in ultraviolet light. Lacquer is used in commercial finishing of wood furniture. It is very fast drying, but also very flammable. It is applied with a spray in a spray booth designed to keep dust particles out. Lacquer is applied in many thin coats with extremely fine sanding between coats. Lacquer provides a durable and water-resistant finish.
Polyurethane (which is a plastic) can be classified as a varnish or a lacquer, although it is neither. Polyurethane varnishes are resistant to water, alcohol, and fingerprints. They cannot be used over shellac or paste wood fillers. They can be either clear or pigmented, matte or gloss, but they will eventually lose their gloss. Urethanes provide better abrasion resistance than varnishes or lacquers do.
Decorating Gallery 10.6 Dark Stain Is Being Applied on Oak Flooring. Taiga/Fotolia
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Shellac is a natural resin secreted by the lac bug. The resin is harvested from tree branches, processed, and dissolved in denatured alcohol. It is available in clear, orange, and pigmented white. Orange shellac is used for restoring antique furniture and floors. White shellac is sometimes used as a stain killer on raw wood. Shellac is an old and natural finish with low toxicity. On furniture, shellac turns white when water stands on it; therefore, coasters are necessary to protect the wood.
Stains are used when one wants to change the color of the natural wood but still wants the wood grain to show. Stains penetrate the surface of wood, adding oils to help preserve the wood. A stain can be either semi-transparent, adding color while allowing the texture of the wood grain to show through, or solid color, adding color and covering most of the grain pattern. Stains do not keep moisture out completely as paints would. For indoor use, a varnish or lacquer is often applied over a stain on wood to protect it.
Varnish is a mixture of resin, oil, dryer, and solvent. Older formulas for varnish used oleoresinous varnishes that tended to yellow over time. Varnishes for interior use usually include alkyd resins.
Alkyd varnishes have good initial color retention, but may crack and peel over time. Spar varnish is a varnish with additives that give the material a resistance to salt water, for use near the ocean. Varnishes that contain ultraviolet-screening agents exhibit less discoloration and generally greater durability. Flat and satin varnishes have an agent such as silica added. They do not have the reflective quality associated with gloss varnish, and they are less abrasion resistant.