Cleaning Up the Confusion About Clear Wood Finishes
When it comes to clear wood finishes, nothing is really very clear. Terminology is frequently misused or used interchangeably. Sometimes an advantage of one type of product over another is so subtle as to seem unimportant, but it can be distinct enough to really matter in some applications.
by Susan M. Brimo-Cox
It’s no wonder, then, that selecting the correct clear finish for the job can be a real puzzler, even for the professional.
Let’s start with the lingo: Varnish. What is it? According to the Paint Quality Institute, varnish is “a liquid composition that transforms to a transparent solid film after being applied in a thin layer.” But there’s more to it. Ronnie Holman of Flecto says that in old-formula varnishes the oils, such as linseed or soybean oil, are cooked and modified with resins to make them harder and dry faster. Bob Flexner, author, editor and long-time refinisher, reports these alkyd resin products have been around since the 1920s, when they were formulated using sap (also called resin or rosin) from pine trees. The more oil used, the softer the finish. The more resin, the harder the finish.
With modern chemistry came synthetic products; and most resins used today are synthetic. It’s not a bad thing. Using a commodity product, such as natural resins, drives the price up. Synthetics mean manufacturers can make more product at a lower price.
Modern chemistry also gave us polyurethanes. Mike Williams, of Marketing at Flecto, says that in the 1950s Varathane was the first polyurethane introduced on the consumer market. (Sometimes the Varathane brand name is used incorrectly as synonymous for polyurethane, like Kleenex is often incorrectly used to mean any facial tissue.) Polyurethanes are varnishes—made with a mixture of urethane-alkyd resins. “Polyurethanes typically use the toluene diisocyanate modifier. It’s what adds wear and abrasion resistance to varnishes. It adds hardness and resilience,” explains Holman.
Shellac is another clear wood finish that uses a natural resin-a secretion from the lac bug. Shellac is made by dissolving this resin in alcohol.
Just like polyurethanes have pretty much taken over the varnish market, lacquer overtook shellac in its category. Lacquer, also solvent-based, is commonly made with nitrocellulose, which comes from plants. According to Tom Barnum of Deft Incorporated, the professional finishing industry is the largest user of clear finishes: cabinet and furniture manufacturers, paint contractors and the antique restoration/refinishing industry all consider lacquer the finish of choice.
The recent newcomers to the clear finish market are water-based products. Don’t let the names fool you, however. Water-based polyurethanes, water-based varnishes and water-based lacquers are not the same as the original products. For example, explains Holman, “water-based floor finishes are typically blends of products compounding acrylic emulsions with polyurethane dispersion-a poly resin converted from solvent-based to water-based.” A more elementary explanation, he says, is its almost like taking a dry polyurethane film, grinding it up and putting it in a water system with just enough solvent to melt the particles together to form a continuous film.
There are other clear wood finish products. True oil finishes-either linseed or tung oil-usually enhance the beauty of wood, but do little to protect it. Spar varnish is a flexible varnish product that performs well in marine applications, like on boats, but offers little scratch resistance. Wax-typically paste wax-also offers little protection. As a group these products would not be on the professional painter’s shopping list.
Keeping the main categories of clear wood finishes straight may not be easy, even if you know what each of them are. Flexner says how each of the products cures is more important than the resins used and he uses these differences to group the products. How each group cures is also an important consideration for the professional painter.
“Evaporative” finishes cure when their solvents evaporate. Shellacs and lacquers fall into this group.
Varnishes and polyurethanes are in the “reactive” group. Their finishes cure with a chemical reaction when the liquid finish comes into contact with oxygen.
Water-based finishes are “coalescing” finishes because they use both evaporation and a chemical reaction to melt or coalesce together.
Each group of products, and often the products within each group, have distinct advantages and disadvantages.
Shellacs and lacquers dry very quickly and are excellent when dust may be a problem. Both develop a tough coating on wood, though lacquer is more durable than shellac. Because the molecules are small, light can reach through these finishes to give “depth” to wood. And because these products are solvent-based, when applying multiple coats it is not necessary to sand between coats. Barnum explains, “When lacquer is applied over a previous lacquer coat, it partially dissolves that coat and chemically bonds.” This characteristic also makes these finishes easy to repair.
There are disadvantages to the evaporative finishes. Lacquer is highly flammable and emits dangerous fumes. It also typically needs to be applied with a sprayer, which limits its use, though Barnum says Deft was the first to introduce a brushable lacquer. And while lacquer stays clear for a long time, it has a slight tendency to amber over time, according to Barnum. Shellac typically has an orange or amber tint and can color wood, but the tint is sometimes chemically bleached out. Shellac also doesn’t have much of a shelf-life. For moisture and abrasion resistance, this group is good, but reactive finishes are better.
The reactive group products—oil-based varnishes and polyurethanes—have numerous advantages. Varnishes and polyurethanes are very scratch-resistant and chemical-resistant, with polyurethanes being even more durable than the varnishes. Jacquelyn Ferrara of Minwax clear finishes reports, “from a durability standpoint, polyurethane is the most durable for toughness. It wears beautifully, resists abrasion and scuffing. It makes wood surfaces protected and beautiful. It’s good for floors, furniture, cabinetry and molding. It is especially good for heavy-use surfaces.”
All this durability comes at a cost, however. Varnishes, and especially polyurethanes, take a long time to dry. A typical polyurethane takes 24-hours to dry. Professional floor finishers who use oil-based polyurethanes are providing a very durable coating, but they usually have to apply one coat one day, the second coat the next day and the third coat the day after that. If you can’t control dust, this can be a real problem. Also, polyurethanes don’t stick well to other finishes or themselves. Sanding between coats is a must and repairing these finishes is not easy. When applying varnishes and polyurethanes you also need to watch out for runs and sagging. Another potential problem is that these finishes yellow over time, though, according to Williams, from an esthetic perspective, the yellowing adds a patina to wood, “warming” certain woods, like oak.
The water-based “family” has its own distinctive benefits. Dan Greenwell of Duron Incorporated says, “The advantage of water-based over solvent-based products is that they are water-clear, have low odor and are quicker to dry.” Faster drying time might be important for some professionals. By using water-based polyurethanes, professional floor finishers can apply several coats in one day (sanding between coats is still required). But while water-based products are moisture-, abrasion- and chemical resistant, they are not as tough as their oil-based cousins. The obvious soap and water clean up, however, is a plus.
The clear color of water-based polyurethanes is definitely advantageous in certain applications. For example, if you’re trying to match a finish, it will be easier to do it if the wood you are trying to match was given a clear water-based finish. The yellowing of a solvent-based polyurethane could be near impossible to mimic. Greenwell says some people want to put a protective clear coat on top of a painted surface, “some water-based polyurethanes can be used as a top coat over paint. It doesn’t change the original color, but adds depth.” Water-based polyurethanes can usually be used over oil or latex-based paint; over lightly stained wood, such as pickling stain; over wall paper, borders and stenciling.
There are several disadvantages to using these water-based products, however. Water-based products tend to bubble when you brush them. Greenwell points out that the water in the product “will raise the grain of the wood far more than any solvent-based product will. The soft grain of oak raises substantially.” Ferrara cautions, when using water-based products, to use sand paper not steel wool when sanding, as steel wool fibers can rust-something you don’t want in the finish. The curing of water-based finishes is also greatly affected by temperature and humidity.
When deciding which product to use, carefully consider all aspects of the job. The wear and tear the finished job has to endure-whether it be a floor, cabinets, doors or molding-helps you select the durability required. The equipment you use to apply the finish is another consideration.
If you can use spray equipment, lacquer or water-based products may deliver what you need. However, keep in mind that water-based products may clog the sprayer and also may tend to dry before the sprayer pushes the product onto the surface. If you have to brush on the finish then oil-based varnish/polyurethanes or water-based products, using natural or synthetic bristles respectively, may be your only options.
What kind of finish do you want? Shellac and lacquer are high gloss. Varnish, polyurethanes and water-based products are available in matte, semi-gloss and gloss.
The professional has several choices as to how he or she buys the product, too, explains Barnum. Products can be purchased “off the shelf,” ready to use. Pre-catalyzed products (lacquer and water-based) offer faster drying times, but have a shorter shelf life of about 12 months. With two-part systems-“conversion varnishes”-you buy two products and mix them before using. The pot life of these products is only about an hour and they are used more by original equipment manufacturers than others, but it is a growing area, Barnum adds.
The majority of clear wood finishes are ideal for indoor use. Their use for outdoor applications is another matter because clear finishes lack the protective benefits of pigment. Varnishes and polyurethanes shouldn’t be used for outdoor applications unless they are specifically formulated for exterior use, and then you should limit their use to small surface areas, such as a front door, Greenwell cautions. “Some are formulated with UV absorbers, which help, but these products tend to be high-maintenance products. UV can result in early failure and (these products) should not be used on broad surfaces such as exterior siding.” There are specific coating products for these applications, he adds. Spar varnishes and spar urethanes may offer more durability, but are usually limited to use on doors and outdoor furniture.
In difficult areas, such as some molding and trim, where applying a clear wood finish with a brush or sprayer is difficult, oil-based wipe-on or gel polyurethane may come in handy. “Sometimes it’s tough to do some areas and intricate moldings. (This kind of product) gives you more control if you don’t need maximum protection,” says Ferrara.
With so many clear wood finishing products available, and new ones coming on the market all the time, professional painters might consider additional opportunities that these products offer. Williams explains, “Virtually anytime a painter is painting a ceiling he removes everything out of the room. It is a great add-on sale to contractors to refinish a wood floor at that time.”
It should be obvious by now that all of these clear wood finishes have advantages, but none are perfect. Each category and type of clear wood finish product is formulated differently to deliver different benefits and results. Each has different application requirements and drying/curing times. Each has different maintenance needs. And each manufacturer may call its products by slightly different names. While it may mean more work up front, make sure you match the right product for each job. Read the labels. You might consider experimenting-buying small amounts of different products to try out on scrap wood. After all, a little time spent before the job can save a lot of time after the job, unless you like call-backs.