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Project Difficulty:
Moderate
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Do-it-yourselfers often get nervous about using clear finishes for wood. They're afraid they'll make a mistake and end up with runs and sags in the finish, dirt, bubbles, or visible brush marks. These things can happen, but even if you do end up making a mistake, fixing it can be as simple as stripping off the finish and starting over again. You can avoid many problems, though, if you choose the right finish and apply it correctly.
Considering the dozens of finishes available in paint stores, it's easy to see why anyone could become overwhelmed by the number of choices (Fig. 1). There are, however, only five types of clear finishes from which to select: oil-based finishes, varnishes and polyurethanes, shellacs, lacquers, and water-based (latex) finishes. Concentrate on the differences in types rather than in brands. Differences between brands are so small that you may not even notice them; differences between types, however, can be quite significant. Read the information on the package label for additional help in selecting the appropriate finish. Many stores also have finished samples so that you can actually see what the finish will look like.
It is also important to consider what you want the finish to do. Do you want the finish to be transparent, semitransparent, or opaque? Do you want the finish to yield the natural look of open-pored wood, or leave a waxlike finish? What about color? Do you want the finish to give warmth to the wood or cause it to look yellowed? Also, how protective do you need the finish to be? Must it guard against the damaging effects of water penetration and water vapor; will it have to resist heat, chemicals, or solvents; or must it withstand the general wear and tear of everyday use?
Another important aspect in selecting a finish is how easy it is to apply. Can it be applied with a brush or cloth, how many applications are necessary, and how quickly does it dry?
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1.
Oil-Based Finishes
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Oil finishes are easily distinguished because they penetrate the wood rather than film-build (build layer by layer) to a hard-cured finish. Varnish, shellac, lacquer, and water-based finishes film-build in layers, cure hard, and consequently, provide better protection than oil. Oil finishes are, nevertheless, generally easier to apply than film finishes. (Fig. 2)
There are two types of oil finishes: those that cure, and those that don't. Oils that cure can be used as finishes because they seal the wood and can produce a fairly permanent sheen. Oils that don't cure either penetrate deeply into the wood, leaving the surface unprotected, or they remain sticky on the surface.
Linseed oil is pressed from the seed of the flax plant. In its raw form, linseed oil takes a week or longer to cure. To speed the curing, manufacturers add metallic driers that make the oil cure in about a day. Linseed oil containing driers is referred to as "boiled linseed oil." Tung oil is pressed from the nuts of the tung tree, native to China. Like linseed oil, pure tung oil is slow to cure, so manufacturers add driers to speed the process. Neither oil finish is particularly protective or durable, but tung oil is generally better than boiled linseed oil. Oil finishes are popular because they are so easy to apply.
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2.
Varnishes and Polyurethanes
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Varnishes are produced by dissolving resinous products in an oil, such as linseed oil. The resins used may be synthetic alkyds, phenolics, or polyurethanes. In fact, a "polyurethane" finish is technically a varnish made with polyurethane resins. The polyurethane resins make this type of varnish a little more protective and durable than most other types of varnish. (Fig. 3)
When there is more oil than resin in a varnish mix, the resulting "spar" varnish is soft and flexible. This type of varnish is best suited for outdoor projects because it flexes when the wood expands and contracts in response to fluctuations in the temperature and humidity.
Varnishes are available in a variety of sheens that range from gloss to flat. Satin finishes are the most popular because they look hand-rubbed with steel wool. If you use a satin-sheen varnish, be sure to stir it well because the ingredient that produces the sheen tends to settle to the bottom of the can.
Varnish is one of the most protective and durable of all finishes--three or four coats offer excellent protection.
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3.
Oil/Varnish Blends
Varnish can also be mixed with linseed or tung oil to create an oil/varnish blend. When this is done, the product is a little more protective and durable than oil alone and still cures to a fairly soft finish. You still need to wipe off the excess finish after each coat. Oil/varnish blends are popular because they are easy to use. Most are sold under names like "Danish oil" and "teak oil." Apply oil/varnish blends just like boiled linseed oil.
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4.
Shellac
Shellac is the only natural resin still widely used as a finish (Fig. 4). Shellac is not as water-resistant as varnish or lacquer, but it is quite resistant. For heavy-use areas such as tabletops and kitchen cabinets, you should consider a more durable finish. If you decide to use shellac, you should always buy it fresh. Older shellac--more than a year or two old--will take longer to dry and will never get as hard. Most cans are stamped with the date of manufacture. Try to find a can that is no more than six months to a year old.
Shellac dries quickly as its alcohol solvent evaporates. This is a real advantage for reducing dust problems. In most cases, the drying occurs so fast that the dust doesn't have time to settle. The disadvantage to a fast-drying finish is that it is difficult to apply using a standard brush or a rag. However, there are ways around this.
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5.
Lacquers
Lacquer is the primary finish used in production work and by professional finishers and refinishers. This is because lacquer dries very quickly, reducing dust problems, and it is easy to apply with a spray gun. The downside of using lacquer is that the solvent fumes left by spraying are toxic, and the residual lacquer dust can be explosive. As a result, few homeowners use lacquer.
There are lacquers that cure slowly enough so that they can be brushed. Although they also employ toxic solvents, these "brushing lacquers" are no more difficult to apply than shellac.
Though more expensive, you can purchase lacquer in aerosol spray cans convenient for small projects.
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6.
Water-Based Finishes
Due partly to stricter air-quality laws, a new market has been created for water-based finishes. These finishes are often marketed as "varnish," "polyurethane," and "lacquer," making it difficult to distinguish them from their solvent-based cousins. However, you can always tell a water-based finish by the clean-up instructions on the can. If you can clean up with water, it's a water-based finish.
Think of water-based finishes as essentially latex paint without the pigment. Like latex, water-based finishes are easy to clean up and don't have a strong solvent odor. However, water-based finishes may produce pronounced brush marks and aren't as durable as solvent-based finishes. Comparatively, water-based finishes don't produce as level a coating as oil-based finishes, and they are also much less resistant to heat, solvent, acid, and alkali damage. In addition, water-based finishes bubble at least as badly as oil-based varnishes, but the bubbles are less likely to deflate. Some manufacturers offer a solvent that you can add to a water-based finish that will reduce bubbling and improve flow, but it is not stocked in most stores. Adding a solvent also defeats the primary reason for using a water-based finish--environmental safety.
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