Walk into any hardware store and the array of finishes will confuse the average person. There are those made for floors and others for furniture, some for interior, others for exterior use, some are resin based and employ a chemically reactive curing process, while others are fast drying lacquers or even water based.
The ideal wood finish would penetrate deep into the surface dry quickly, provide good abrasion and stain resistance, rub out easily and look great. Also, you would want the option to apply this finish with a rag or brush or spray gun.
Finishes come in two basic formats: those that dry through evaporation and those that cure by a chemical process. Those that evaporate, typically water or acetone based suspend the solid particles in the liquid. As the liquid evaporates the suspended solids harden, creating a smooth finish. These finishes are not usually designed nor recommended for external use. The finishes that cure are usually oil-based resins that take longer to dry, but often produce a superior finish. Unfortunately, no one finish has all the properties, but oil based finishes come pretty close.
All oil based finishes have one thing in common: In its liquid form most of the finish molecule is composed of vegetable oil. These finishes are made by chemically combining a modifying resin with a vegetable oil to produce a finish molecule that is a liquid when applied to the wood surface but solid after it cures to an impervious film in a short period of time. The two most common types of vegetable oil used in furniture grade finishes are Linseed and Soybean Oil, but occasionally specialty varnishes may use Tung Oil. The type of oil used in a varnish resin has less effect on the finish properties than does the amount of oil used. Today's chemist have many sophisticated chemicals at their disposal to subtly change the character of a finish during the cooking process. But the two most popular products available today are medium-oil Polyurethane and longer lasting and more resilient Spar Varnish.
So how do the professionals select a finish to restore your door?
The ratio of oil to modifying resin is known as "oil-length" in industry term's... It determines the flexibility of the dried film, curing or drying time and application method. For commercial wood applications, there are two primary finishes, long-oil and medium-oil generically named "Varnishes". Long oil's typically take two days or longer to cure between coats and applying a four-coat finish could take two or more weeks to complete a door project, hence are not suitable for door refinishing. Medium-oil finishes, which are the backbone of today's Polyurethane and Spar Varnishes, are faster curing and take between 6 and 24 hours between applications. On applying the final finish coat it is best to allow one or more days before using the finished product.
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