How Does Varnish Work?

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Protecting Wood

  • Varnish seals wood by filling pores and grain. Varnish is made by combining resin, the sticky substance in plants with oil like linseed, tung, or walnut oil and a solvent such as turpentine or paint thinner. The polyunsaturated fatty acids in the oils oxidize and form new compounds with the resins resulting in a tough but flexible skin over the surface of the wood. The solvent keeps the compounds in suspension until the varnish is laid on and the solvent can evaporate, leaving the oil and resin to "cure." Since varnish "cures" using oxidation rather than using evaporation to "dry," the oil has time to seep into the pores of the wood and moisturize the wood itself. The result of a thin layer of varnish, properly cured, is a rich look and a surface dotted with bumps of raised grain. When the first coat is lightly sanded to remove the raised portions, a second thin coat will bond by dissolving part of the first and curing again as a whole.

Wearing Over Time

  • Varnish protects finishes as it wears. Most varnishes wear off over the years or dry from exposure, causing crazing. As it wears, the varnish takes the abuse, absorbing impacts and ultraviolet rays, protecting the surface beneath it. When the varnish wears off, or alligators, it can be easily renewed or replaced. Turpentine or paint thinner (depending on the resin and oil used in the varnish) will soften the varnish, "cleaning the surface for recoating with a few fresh coats of varnish. Paint remover will soften it for complete removal. Because it protects so completely and is so easy to remove, varnish is used by conservators for fine art, much of which was originally protected with varnish by the artist.

As a Base Coat

  • Varnish provides a good base for other finishes. Because varnish seals so well, is gentle on surfaces and is easy to remove, it is used as a base for other finishes like wax or paint. Removal of these finishes would require sanding to get all of the material out of the wood. Since varnish seals the wood, these finishes are easily removed, leaving the wood clean and ready for a new finish with only a wipe-down using turpentine or paint thinner. Varnish is not as time-consuming a process as rubbed linseed or tung oil finishes and can have a higher gloss than wax. Unlike polyurethane, which can delaminate from the surface of wood in sunlight, an oil-heavy natural varnish can work well in exterior applications.

  • Photo Credit DRW & Associates, Inc., Wikimedia Commons
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