SECRETS OF PAINT AND STAIN CHEMISTRY

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VARNISH

Technically, all resin-and-solvent mixes are varnishes; shellac is a spirit varnish—it uses distilled spirits as a solvent; spar varnish uses a drying-type vegetable oil as a solvent; enamel is a varnish containing pigments; lacquer is a demanding finish that uses a binder made from the natural polymer cellulose derived from wood (it lets tree limbs bend in the wind) or cotton.

In common usage today, "varnish" means a mix of drying oil and a natural or artificial resin that is cooked (often with an inert-metal cata lyst) to make a clear finish. It may use a sol vent that is a little more noxious than shellac's, but the fnal coat is usually waterproof and/or tough and long-lasting. For cen turies, varnish has been made from resins collected from tree saps and dissolved in vegetable oil such as linseed oil (extracted from flax seed), and thinned with turpentine—a volatile, fluid oleo resin steam-distilled from heart wood of pine trees (leaving solid rosin, that makes good varnish when mixed with a drying oil).

If you want to spend the time, you can slash the bark of conifers in the spring and collect the sap as it oozes out and dries in soft, sticky blobs. Dissolved in warm turpentine, strained like Lac bug scales, cooked with lin seed oil and applied in thin coats, it makes an inferior but us able clear varnish. In home-mixed varnishes fillers and putty, always use boiled linseed oil; the raw variety is sold, but will never dry.

MODERN VARNISHES AND PAINTS

Today, synthetic resins derived from coal tar and petroleum are replacing insect secretions and tree saps as binders, and petroleum distillates are replacing vegetable oils as solvents. Not all of them pose a serious hazard to air quality or human health.

Some traditional varnishes combine natural and artificial ingredients. One such, spar varnish, is waterproof and was originated for use on "bright work," the natural wood-colored parts of boats that are exposed to ravages of saltwater and continual sunlight. Conventional mixes will protect the wood and retain their clarity and gloss for a year before becoming chalky. More modern varnishes containing ultraviolet barriers and a more sophisticated mix of ingredients will last up to three years. And finishes containing resins with such newly familiar names as polyurethane, epoxy/amine, and phenolitic constitute a whole new class of finishing products that promise to last for many years. The chemistry behind these technical-sounding names is complicated. But the end product is nothing but a more durable sheetplastic formed in place—just like when you dissolve hardened tree sap in a drying oil, spread it on a flat surface, and let it dry.

Some examples, Interlux's Schooner combines phenolic resin and tung oil, and their Clipper is a longer-lasting one-part polyurethane; both contain UV protection. Detco's Crystal Varnish is another UV-protected tung oil and phenolic-resin mix, while their Sterling linear polyurethane coatings are two-part colored finishes that are demanding to mix and apply, but will last for many years.

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