The natural beauty of wood can be mesmerizing, with the lines and swirls of the grain drawing us in. Anyone who has worked with wood has developed an appreciation for its natural beauty. That beauty can be brought out by the right application of the right finish, enhancing the colors to make the grain even more prominent. Selecting the right finish and applying it in the right way are essential to maximize its part in the overall beauty of the finished project.
There are several finish materials we have available to us to choose from. Yet many people have a habit of referring to all of them by the same name, whether that be “lacquer” or “varnish.” But lacquer and varnish are different materials, as are polyurethane and shellac as well. While they may all provide similar results, they each have their own place, with their own advantages and disadvantages.
Before the 1960s, shellac was a favorite finish, used extensively by furniture manufacturers. That has since been replaced by lacquer and even more recently by polyurethane. Each of these finishes has affected the overall look of the furniture from its own era. To understand the effect, as well as the effect on our own work, we must understand the difference between these materials.
Shellac is perhaps the most interesting of these finishes, in that it is the dried secretion of the female lac beetle. This secretion is normally applied over the cocoons for its larvae in order to protect them. Collected, it is suspended in an alcohol-based solvent. This makes shellac very fast-drying, as the alcohol evaporates quickly. This fast-drying capability allows shellac to work well at sealing porous woods.
Another way that shellac is used is for hiding stains. Bin primer/sealer is actually white tinted shellac. When applied over stains in painted wood or drywall, the fast dry time of the shellac, along with a heavy white pigment content seal in the stain and provide a good base for the application of paint. However, this clearly doesn’t work as a wood finish, since it would cover the grain’s appearance.
The use of shellac is where the idea has come from that leaving a coffee mug or glass of iced tea on a table will leave a white ring. It is shellac that this happens with. Shellac naturally has an orange tint to it, which affects the color of the wood. It can also be tinted to a wide variety of colors.
Lacquer is a clear nitrocellulose dissolved in a solvent. When the solvent dries, it leaves a crystal-clear finish. It is self-leveling, so when applied by spray, lacquer results in an extremely smooth finish, without orange peel. That allows it to provide the highest gloss finish of any clear wood finish. Lacquer can also be tinted, which has led to the colored lacquer finish (especially black) that is associated with furniture from the orient. Interestingly enough, nail polish is actually colored lacquer.
The natural finish of lacquer is high-gloss, although it is possible to buy it in semi-gloss, and satin finishes. Much of the pre-packaged “metal paint” like Rust-Oleum is actually lacquer. It provides a durable finish that is easy to apply.
One of the big advantages of lacquer over any other finish is that when it is necessary to make a repair to the finish, all that’s required is spraying another coat on the affected area. The lacquer’s solvent will dissolve some of the existing finish, allowing the old finish and new finish to meld into one, hiding any spray lines or edges.
Of all these finishes, lacquer has the thinnest viscosity, requiring the application of more coats. Not only is a paint sprayer required, preferably a high-volume, low-pressure one (HVLP), but due to the solvents, an excellent ventilation system is required. Dried lacquer is the most impervious to solvents, including alcohol.
Today, the term “varnish” has become rather universal, referring to pretty much any clear wood finish; but there is an actual material called “varnish,” made from resins, oils and solvents. This differs from polyurethane, although polyurethane is often called “varnish,” even on the label.
Due to its makeup, oil-based varnish, sometimes referred to as “spar varnish” is the most weather resistant material on this list. It does tend to yellow the wood slightly, which some people like. Exposure to UV light can increase the yellowing of the varnish, as well as cracking the surface if applied insufficiently or not allowed enough time to dry between coats. True varnish needs a full 24 hours of dry time between coats.
Oil-based varnishes have largely been replaced today by polyurethane, although the two are different materials. Polyurethane is a plastic and arguably provides the most durable finish of any finish mentioned in this article. Both oil-based and water-based polyurethanes exist, although the oil-based is more durable. It is also the easiest to apply and is available in sheens ranging from high gloss all the way down to flat.
As with vanish, oil-based polyurethane has a slow dry time, with the water-based version drying much faster. The water-based is also convenient in that it is considerably easier to clean up. Many woodworkers use foam brushes for applying oil-based polyurethane and varnish to avoid cleanup altogether; but a better finish result is achieved when natural bristle brushes are used. Another option is to purchase polyurethane as a spray
As with lacquer and varnish, when using oil-based polyurethane a well-ventilated work area is required. It is recommended to wear a respirator with a chemical filter, to remove the VOCs from the air being breathed. As with varnish, oil-based polyurethane will impart a slight yellowish hue to the wood.
|Shellac||Lacquer||Varnish||Oil-based Polyurethane||Water-based Polyurethane|
|Dry time||1 hr.||30 min.||24 hr.||24 hr.||2 hr.|
|Ease of application||Brushed||Sprayed; self-leveling||Brushed||Brushed||Brushed or sprayed|
|Cleanup||Alcohol||Lacquer thinner||Mineral spirits||Mineral spirits||Water|
Notes on Application
Always apply any of these finishes in as dust-free an environment as possible. That can be tricky in a woodworking shop, where dust is normal. If space allows, the installation of a paint booth goes a long way towards providing a dust-free environment for finishing operations.
Theoretically, all of these finishes can be brushed, rolled or sprayed. However, lacquer is almost always sprayed, so as to allow the finish to level out for an ultra-smooth finish. The only way I have seen shellac available in a spray is in the form of Bulls-eye stain killer. varnish and polyurethane can be sprayed, but it should be noted that the version canned for spray has a lower viscosity, with a lower content of solids. This results in a much thinner coat.
When brushing, a natural bristle brush, often called “china bristle,” is best for oil-based products and a nylon brush is best for water-based products. Disposable foam brushes can be used, but will actually not give as good a finish. Synthetic rollers will work with all of the materials and rags can be used with oil-based varnish and polyurethane for a hand-rubbed finish.
Most of the time, multiple coats of finish need to be applied, with light sanding with a fine sandpaper between coats. This sanding performs two purposes: scuffing the surface for good adhesion and removal of bumps caused by dust getting into the finish. On lacquer, sanding can also be used to remove drips and runs.
Two coats are generally enough for most applications; however, it is not uncommon for fine furniture to have multiple coats, usually hand-rubbed. The thicker finish creates a visible difference, as well as a surface that is smoother to the touch. This has become one of the signs of fine furniture.