The selection of a protective clear finish for your woodworking project is critical to the look and durability of your handiwork. Many woodworkers don’t give the final finish much thought and will reach for the nearest can of polyurethane varnish. Although polyurethane can be an excellent choice in wood finishes, there are many other options that may be more appropriate for your circumstances. If you do select polyurethane, you also have many choices. Do you want the sheen to be high gloss, semi-gloss, or satin? Wipe-on or brush on application? Water-based or oil-based?Selecting the proper clear finish depends on two main considerations: the desired look of the piece, and its intended use. You would not want to use shellac on the family’s kitchen table, which gets heavy abuse from hungry kids banging dishes and spilling syrupy drinks. Shellac simply won’t hold up to that kind of use. On the other hand, you shouldn’t use high-gloss polyurethane on a late-1800’s bedroom dresser. It would certainly protect the furniture, but would detract from the antique patina because the gloss simply isn’t appropriate for a piece from that period.
Although there are wrong choices for a clear finish, there usually isn’t one right choice for a given project, either. Unless you are restoring an antique to its original condition, the choice depends on aesthetics and ease of use, in addition to the previously mentioned considerations of appearance and durability.
So, how do you choose? We’ll take a look at the characteristics of several readily available clear protective finishes and evaluate their appropriateness for various types of wood projects. Then, before committing to a specific product, you may want to test out two or three choices on a hidden area or on a piece of wood similar to that of which your project is made. Compare the looks of the different products after they’ve dried, and with the final number of coats that you’ll be using.
Types of Finishes
Protective finishes generally fall into one of five categories: shellac, lacquer, varnish, oil, and water-based.
Shellac is the oldest of the modern finishes and is still made from the resin secretions of the lac beetle, found in India and Asia. After collecting the resin, it is crushed, washed, and dried. The material is then cleaned and heated, and processed into flakes. The dry flakes are dissolved in denatured alcohol for final use as a protective finish.
Shellac can be purchased in prepared form, but it has a short shelf life of only a few months. I’ve found that prepared shellac is not widely available in the large home centers, except in spray cans, but is easier to find at a local hardware store or woodworking center. If you purchase prepared shellac, be sure to check the date of manufacture, which may be printed on the bottom of the can; don’t purchase it if it won’t be used within six months of the manufacturing date.
Prepared shellac is sold in various “cuts” that indicate the weight of dry shellac dissolved in a gallon of solvent. For example, a one-quart can of 1-pound cut means that a quarter-pound of shellac has been dissolved in a quart of solvent. If you do purchase prepared shellac, you may need to cut it with denatured alcohol to bring it to the correct weight. A 2-pound cut can be made into a 1-pound cut by simply mix equal portions of prepared shellac and high-quality denatured alcohol.
If a longer shelf life is important, purchase the dry flakes and mix up only the amount that you will need for immediate use. There are many mail order suppliers of dry shellac, and you can also find them at woodworking specialty shops. Complete directions for dissolving the flakes solvent should be provided with your order.
Shellac does not provide a completely clear finish, but has an amber tint that gives a mellow depth to the finish. It does not have any plasticizers or other synthetics that give some finishes a characteristic artificial appearance. Each coat also melts into the one below it, as opposed to some other finishes that layer atop one another. This means that a shellac finish is easy to repair if it is scratched or dinged. Shellac also dries very quickly, so dust settling into a wet finish isn’t a big problem, and shellac can be recoated in a very short period of time. Several coats can be applied in a single afternoon if necessary.
Another advantage to shellac is that it acts as a sealing coat and is not repelled by contaminants in old wood. If you have ever tried to refinish an old table that had been cleaned for years with silicone-containing dusting sprays, then you know that it is very difficult to achieve a consistent finish with polyurethane varnish. Shellac can be used as a final finish, or as a sealant below other clear coats.
So why would you not want to use shellac? One of the most common reasons is the short shelf life that was discussed earlier. Many refinishers simply find it easier to open a can of poly than to mix up a fresh batch of shellac. Shellac is also less resistant to staining and the formation of water rings, so – as stated earlier – it’s not the best choice for the breakfast table of an active family.
But if you aren’t worried about stains and water marks, and you don’t mind the little extra work involved in preparing shellac, then it’s hard to beat the beautiful, lasting glow from a few coats of shellac.
If you’ve ever purchased commercially-manufactured furniture, it more than likely has a lacquer finish. For the professional wood finisher, lacquer is tough to beat. It dries to a very clear finish in as little as 15 to 30 minutes, so several coats can be applied in a short amount of time and dust is not a big problem. Catalyzed lacquer is very durable and resists scratches, staining and water rings.
Lacquer must be applied as a spray finish. A professional spray setup can require a large investment, but high-volume, low-pressure spray systems are available for the home user and part-time refinisher. The spray equipment must be carefully maintained and controlled to provide a smooth, consistent finish. Improper control of air and fluid pressures or moisture in the air lines can cause uneven or rough surfaces, pin holes, graininess, and other problems.
There are several types of oil-based varnish, most made with vegetable oil and resin. All share the characteristics of durability and resistance to moisture and stains. Modern synthetic varnish types include alkyd, phenolic, and polyurethane. These varnishes have good brushing characteristics and slow drying times, so it’s relatively easy to brush on a good, even finish, even for the inexperienced refinisher. However, the slow drying time that aids the application is also a drawback, as it allows more time for dust to settle into the finish. A clean, dust-free environment is essential for laying down a good finish with oil-based varnish.
Another drawback to varnish is that each coat is distinct from the one below it. Each coat must be lightly sanded so that the next coat adheres properly. This layer effect also makes it difficult to sand out blemishes in the finish: if you cut through one or more layers, a thin line will show along the boundary between the layers.
Still, you can’t beat varnish for its resistance to wear, moisture, and heat. It makes an excellent finish for furniture or other items that receive a lot of use – or abuse. Remember that most varnishes will give a slight amber cast to the wood, and tend to yellow over time, so varnish is best for stained or dark wood, not as a protective finish over white or very light-colored wood.
If you are varnishing wood for outdoor use, be sure to use a high quality spar varnish. Spar varnish is a type of phenolic varnish that is formulated to be more flexible than indoor varnishes, so it is more resistant to wood’s expansion and contraction in the extremes of outdoor temperatures. Many brands of spar varnish also contain UV inhibitors to protect the wood from sunlight.
Unlike the other finishes discussed in this article, oil finishes do not form a protective layer atop the wood. Rather, they penetrate into the wood fibers. Oil finishes are very easy to apply: simply wipe on with a rag, allow some time to penetrate, then wipe off the excess. Several applications are usually needed, and it’s a good idea to reapply once or twice a year as the oil dries.
Because the oil does not form a thick topcoat, it won’t offer much protection from heavy use. However, the penetration of the oil tends to intensify the natural beauty of the wood. An oil finish is a good choice for furniture that won’t see a lot of abuse; bedroom furniture or antiques that are used for display rather than daily use are good candidates.
Your choices in an oil finish include linseed oil, tung oil, and oil-varnish mixtures that are commonly marketed as Danish Oil or Teak Oil. If you use linseed oil, you will probably want to use the boiled version, which includes dryers to reduce the naturally slow drying time of the oil.
Tung oil is a bit more durable than linseed oil, and may not require annual recoating unless the finish becomes noticeably dull due to wear. Tung oil dries more slowly than boiled linseed oil, and still requires the application of several coats with rubbing between coats in order to build up a suitable finish.
If you are concerned about the high VOC content of traditional varnishes, or if you are extremely sensitive to these vapors, you might want to consider a water-based finish. Water-based finishes are less toxic and less combustible than their oil-based predecessors, so they are also good choices for children who are woodworking hobbyists. Water-based finishes may also be preferable if you do your finishing near the living area of your house. The water-based products won’t produce the same vapor problems, but you will still need to use proper ventilation during application and drying.
These advantages do not come without a price. Water-based finishes can be more difficult to apply. They tend to run and are more likely to show brush marks, and they don’t polish out to a high-gloss that is desired in many finishes.
Protecting the Finish
Wax is not recommended as the only protection on wood because it offers little protection to the wood, and is nearly impossible to completely remove except through deep sanding. However, a coat or two of high-quality paste wax will protect the clear finish and add more beauty to your project. By reducing the slip resistance of the furniture, items are more likely to glide across the finish instead of scratching it. Wax will also add a nice shine to the finish by filling in small scratches in the underlying protective finish.
Look for a high quality paste wax that includes carnauba wax, which is harder and more durable than other types of waxes. The carnauba wax is so hard, though, that it is mixed with softer waxes during manufacturing. Keep in mind that only a very thin film of wax is necessary for the desired results, so don’t worry about buffing too much wax off the furniture after drying.
To keep your furniture protected and in beautiful condition year after year, annual waxing and polishing is recommended.
FOR MORE INFORMATION…
We hope this article has been useful in helping you select an appropriate protective finish for your refinishing or woodworking project. For a comparison of the characteristics, appearance, and durability of many finishes, Rockler has allowed us to reprint their excellent Finishing Comparison Guide, which you can download by clicking here.
This article is copyrighted ©2007-2011 by Thomas M. Roth, Decatur, GA and may not be reused in any form without written permission. For more information, please contact the author at trothatl(at)polyurethanevarnish.com. This is an independent website that receives commission-b6