Finishing Touch: Film finishes
There are two broad categories of wood finishes for furniture and cabinetry. Both are indispensable to woodworking and have their respective advantages. Penetrating finishes, which permeate wood surfaces by means of capillary action, include pure (or straight) oil, polymerized oil and oil/varnish blends. These “rag on, rag off” finishes are the easiest to apply and maintain which makes them particularly well suited for novice woodworkers, and for projects that won’t be subject to a lot of handling (see “Penetrating Oil Finishes,” Dec/Jan 2020). There is also a relatively new class of “hardwax oil” finishes that are similar in application to penetrating finishes but provide a higher level of durability and moisture resistance (see “Hardwax Oil”, Aug/Sep 2018). However, they aren’t the most durable of finishes. For long-lasting endurance, we need to look at the second category of finishes – film finishes.
Film finishes refer to those finishes that build a strong, resilient film on the surface of the wood. These are “brush-on” or “spray” finishes. The three most popular types of indoor brush-on finishes for both professional and hobbyist woodworkers are varnish (including polyurethane/urethane), shellac and waterborne (or water-based). While lacquer is typically sprayed, there are brush-on products as well. The two-part film finishes (conversion or catalyzed varnish and catalyzed lacquer) are also part of the film finish family, but more typically used by professional finishers and in production shops that use HVLP spraying equipment and a spray booth.
It’s common to both spray and brush on film finishes. There are pros and cons to each approach, so do your research before deciding on an option. (Photo by Old Masters)
Brushing is Common
Brushing on a film finish is a very common approach in a small shop. As long as you can keep the dust down, it offers a simple, cost-effective way to apply these types of finishes. (Photo by Old Masters)
Varnishes are durable and add an amber tone to wood. Oil-based varnishes are the most durable, though there are more water-based varnishes on the market today due to tightening environmental regulations. (Photo by General Finishes)
Overlooked by many woodworkers, shellac has a lot going for it. It’s easy to apply with a pad, protects against moisture and is easily repaired. (Photo by Wood Essence)
Though most people spray lacquer in industrial settings, hobby woodworkers can wipe on some special formulations of lacquer if they don’t have spray equipment. (Photo by Old Masters)
Increasing in popularity over the past few decade,s waterborne finishes have a lot of pros. They are generally quick to dry, easy to apply, are quite durable and have low VOCs. (Photo by Saman)
Varnish – durable, dependable, long-lasting
The recipe for a varnish consists of oil (such as linseed, tung or soybean oil), resin (usually a synthetic acrylic, alkyd or polyurethane resin, and rarely a natural resin, such as copal or rosin), solvent (such as naphtha, acetone or mineral spirits) and one or more additives that can include dryers, anti-skinning agents, flattening agents, ultraviolet stabilizers and anti-oxidants.
Go into any hardware store and you’ll likely see an array of “varnish” for indoor use: oil-based varnish, waterborne varnish, oil-based polyurethane, waterborne polyurethane and waterborne oil-modified polyurethane. It might seem confusing, but they’re all varnishes, though made from different recipes.
The proportion of oil in the recipe has a bearing on how durable and flexible the cured finish will be. Using less oil makes for a harder, more durable finish, but it will have less elasticity. The type of resin affects the durability and stability of the finish. Synthetic resins, particularly polyurethane, are the hardest and most durable.
Oil-based varnish usually contains an alkyd resin, waterborne varnish contains acrylic, while all the polyurethane varnishes contain a polyurethane resin (but may have some other resin as well). The polyurethane resin adds greater scratch and heat resistance, but makes the finish less flexible. All the oil-based varnishes impart an amber tone to wood that tends to darken somewhat over time. However, oil-based polyurethane usually contains soybean oil, which imparts a lighter amber tone to the finish.
Anything that is water-based will dry clear and much more quickly than an oil-based finish (making the finish less susceptible to dust contamination), have less of an odour during application, have a low VOC (volatile organic compound) content and be somewhat less durable than its oil-based counterpart.
All the varnish finishes are very durable and provide good resistance to wear, moisture, stains and heat. They’re available in various sheens from flat to gloss. The hardest of these finishes is oil-based polyurethane which is why you most often see it promoted as a floor finish. If you plan to rub out the finish (see “Rubbing the Finish,” Oct/Nov 2005), an oil-based varnish is a better choice, as the alkyd resin makes the finish more flexible.
Spar (or marine) varnish is manufactured for outdoor use. It typically contains a higher oil content and somewhat lower resin content (often a phenolic resin) which makes it more flexible. It also contains UV blockers.
This is any varnish that has been diluted with mineral spirits. You can buy a commercial blend or make your own using a non-critical ratio of 1:1 or 1:2 (mineral spirits to varnish). It makes the varnish easier to apply and quicker to dry. The more coats you apply, the more durable the finish will be once it’s cured. For projects that won’t be handled a lot, four or five coats should suffice. Otherwise, a dozen or more coats aren’t out of the question.
These finishes are sold under names such as Danish oil or antique oil finish. Essentially, they consist of varnish mixed with a penetrating oil (boiled linseed oil or tung oil) and mineral spirits. You can make your own using a non-critical ratio of 1:1:1. An oil/varnish blend is more durable than wiping varnish but takes a bit longer to dry.
Sometimes product names can be confusing. “Polycrylic,” a brand name owned by Minwax, is a waterborne finish comprised primarily of acrylic resin. “Varathane,” a brand name from Rust-Oleum, consists of a product line that includes oil-based and waterborne finishes, stains and conditioners. Tried & True’s “Varnish Oil,” which contains polymerized linseed oil and modified pine sap, is strictly not a film finish, but a penetrating oil finish.
Shellac – develops a beautiful patina over time
There seems to be a certain mystique associated with shellac. Perhaps it has to do with “French polish,” the traditional technique for applying shellac. Or the smooth gloss-like finish that gives an exceptionally deep luster. Whatever the reason, shellac is a versatile finish made from a natural resin secreted by the female lac beetle. The resin is processed into flakes that are dissolved with alcohol to form a fast-drying, moderately wear-resistant finish with excellent water and moisture resistance, but susceptible to damage from alcohol and heat.
Shellac has a short shelf life (whether premixed or mixed in your own shop). Fortunately, it’s easy to dissolve shellac flakes as needed with alcohol (isopropanol, methyl hydrate or ethyl alcohol) in a mixture called a “cut.” A typical one-pound cut consists of one pound of flakes dissolved in one gallon of alcohol (or any proportionate ratio, such as 1/4 pound to one quart). Flakes are available in a range of sheens, from clear (super blonde shellac) to rich, darker reddish brown (garnet shellac). Choose dewaxed shellac especially if you plan to top coat the shellac finish with polyurethane or lacquer, as neither will bond well to waxed shellac.
Shellac is easy to apply and repair, and buffs (or rubs out) to a wonderful finish. You don’t have to sand between coats of shellac unless there are bumps caused by dried dust on the surface. If you need to colour-match the wood, you can add an alcohol soluble dye to shellac or you can stain over the shellac.
Lacquer – unsurpassed clarity, quick drying, super hard
Lacquer has been a preferred finish in production shops for decades because it dries so fast when sprayed, can be easily buffed to an even sheen, takes pigments and dyes very well, and is easy to repair. While brushing lacquers are available, they’re not nearly as popular as the other film finishes.
There are nitrocellulose, acrylic and waterborne lacquers. Most brushing lacquer is made from nitrocellulose (a celluloid made from cellulose and nitric acid), solvents (naphtha, xylene, toluene and others) and plasticizers that make the nitrocellulose less brittle.
Like waterborne finishes, lacquer dries quickly and crystal clear. Unlike most other finishes, lacquer is self-levelling, which makes it somewhat easier to brush. As with shellac, there’s no need to sand between coats, it rubs out to a wonderful luster and is easy to repair. While lacquer is a durable finish, it’s less resistant to scratches, heat and solvents than polyurethane.
You should always wear a respirator equipped with an organic vapor cartridge when brushing lacquer or cleaning your brush with lacquer thinner, as both have a high VOC content.
Waterborne – transparent, quick drying, durable
Waterborne finishes have become very popular over the past few decades, in part because they dry clear and don’t yellow much over time (though some include an amber cast that mimics the traditional oil-finished look). They dry very fast (with little time for dust to dry on the wood surface), clean up quickly with water, and provide very good wear, water and water vapour resistance. However, because they dry so fast, they’re somewhat more of a challenge to brush than varnish. You can add pigmented dyes directly to waterborne finishes, or stain or dye the wood before applying the finish.
You might get a bit confused when you see the term “waterborne” combined with “varnish” or “polyurethane” on the same container, as with Saman’s “Waterbased Varnish.” Waterborne finishes are coalescing finishes. Similar to oil-based finishes, they contain a resin (acrylic, alkyd or urethane) along with a much smaller amount of solvent and water as a carrier. After you lay on a coat of waterborne finish, the water evaporates more quickly than the solvent. The solvent softens the resin so that it bonds into a continuous film.
Depending on the type of resin used, waterborne finishes may be referred to as “water-based varnish,” “water-based lacquer” or “water-based polyurethane,” or simply as a “water-based” wood finish.
Waterborne finishes do raise the grain of wood, usually right after the first coat has been applied. Following coats don’t tend to raise the grain at all. Light sanding between each coat abrades the surface for the next coat to go on more smoothly. Along with shellac, waterborne finishes are the safest to use—they have negligible VOC levels, no toxic additives, are virtually odour free and are non-flammable. And, as environmental restrictions become tighter, it’s likely that waterborne finishes will become much more popular in the future. (see “Waterborne Finishes”, Oct/Nov 2019)