DO-IT-YOURSELF : Polishing Up Wood With Final Finishes

When working with wood, the final finish is critical to the appearance of the piece. Here’s how to choose and apply the wide choice of available wood finishes.

Types of wood finishes

There are two categories of wood finishes:

* Surface finishes harden on top of wood, forming a protective finish. Additional layers adhere mechanically to the “tooth” of a sanded surface. These finishes include polyurethane, lacquer, shellac and varnish.

* Penetrating finishes (tung oil or Danish oil) are absorbed into wood and harden within the fibers. Subsequent applications bond chemically, melting together and soaking into the wood.

Caution: When finishing any wood, always work in a well-ventilated area and wear a cover for your mouth and nose as well as rubber gloves.

* Polyurethane is durable, fast-drying and easy to apply. It comes in satin (matte) and high-gloss formulas. Generally, polyurethane has an amber or yellowish cast that enhances wood’s natural color. It can also be tinted with Japan colors or an oil-base stain. Use turpentine or mineral spirits to thin it.

Although it is sometimes called varnish, polyurethane is not a true varnish, which traditionally is made from distilled natural resins and oils. Spar varnish (urethane) is the best choice for marine or exterior work because it is highly resistant to abrasion, water seepage and sun damage. However, it has a tendency to yellow with age and exposure to sunlight.

* Shellac comes in two forms: flaked and liquid. Both are dissolved in denatured alcohol to the desired “cut.” A three-pound cut--three pounds of shellac dissolved in one gallon of denatured alcohol--is recommended for furniture. When mixed, all shellac has a short shelf life. Mix just enough flaked shellac for your immediate purpose. Premixed shellac should be used within six months.

* Lacquer, because it dries so quickly, is usually sprayed rather than brushed on. As with any finish, several thin coats are better than a single thick coat. Whether spraying or brushing, apply each coat heavily enough to spread and blend together, but not so heavily that you flood the surface. If spraying, rehearse on scrap wood and aim for even coverage without lap marks or drips. If brushing, work along the grain with a good, natural-bristle brush.

Caution: Lacquer and its spray are flammable and toxic. Construct a booth to collect the over-spray of work outside. Always wear goggles and a disposable spray-paint respirator mask. Use water-base lacquer, which is non-flammable and nontoxic, if possible.

* Danish oil is composed of penetrating oils and resins. Danish oil is easy to apply or repair. As the oil soaks into the wood, it reacts with oxygen in the air and hardens within the wood’s fibers--a chemical reaction called “polymerization.” If the finish is damaged, invisible repairs can easily be made by sanding the area, reapplying some oil and vigorously rubbing it in.

Danish oil deepens the wood’s color slightly; subsequent applications deepen it more. The more coats you apply, the harder and more lustrous the finish will be.

* Tung oil is a penetrating oil derived from the seeds of tung trees. It is available in three forms: pure, polymerized and a varnish. Pure tung yields a durable surface with a matte finish. Polymerized tung, which is chemically altered by heating, leaves a glossier and even harder surface. Tung-oil varnish looks and acts more like a surface finish and is prone to chipping. Polymerized and pure tung oil are mixed commercially, creating products in a range of hardness and luster. All alter the wood’s color slightly, but these finishes are the most natural-looking.

Applying wood finishes

* When working with any final finish, treat all parts of a single piece identically--back as well as front, underside as well as top--so the wood reacts evenly to moisture changes and won’t warp. Avoid working in cold or dampness, such as in an unheated basement. Allow the piece and the can of finish to acclimate to room temperature before working. Otherwise, bubbles may form on the surface of the piece.

* To color and finish in a single step, use a penetrating oil, such as Danish or tung, or a commercial mixture of stain and polyurethane. With the stain-polyurethane mixture, the final result is not as deep, clear and rich as it is with finishes that are applied in stages.

* Once dry, all finishes can be hand-polished to a lustrous sheen with abrasive powders or with a premixed polishing compound that imitates powder polishing, available in paint or hardware stores. Pumice powder results in a satin sheen. For high gloss, use rottenstone instead of, or after, pumice.

* Protect the final finish with several thin coats of a high-quality furniture wax. Liquid wax is good for carved pieces and areas that receive light wear. Paste wax stands up better to hard use. Let the wax dry before buffing; otherwise you’re just moving the wax around. Avoid silicone and lemon oil. If the piece ever needs refinishing, silicone can keep the new finish from adhering. So-called lemon oil is mostly kerosene and can damage a finish.