Selecting a clear finish for a wood project typically involves trade-offs between appearance, protection, durability, safety, ease of application, curing time, and other properties. No single finish excels in all respects. For example, polyurethane varnish is a very protective and durable finish but it is not the easiest finish to apply and it produces lots of noxious fumes. Also, it is not easily repaired or removed.
To help you choose a suitable finish, we've put together an online utility that allows you to select properties of interest to see how various finishes score relative to each other: Wood Finish Selector However, even with a tool such as this, you still need to determine which properties are most important to you and which are less important. Think of it as a "must have" versus "nice to have" list. If you are finishing a coffee table, you might assign top priority to the protective qualities of the finish (such as water and wear resistance) and perhaps a lesser priority to properties such as clarity, yellowing, and ease of spraying (especially if you don't own spraying equipment). It's all about trade-offs. Wood Finish Checklist To further assist you in arriving at the best finish for the job at hand, here is a series of questions that you should try to answer. What is the intended use? How durable and protective does the finish need to be? How will you apply the finish? What is your skill level? Do you want a natural, close-to-the-wood look? Is clarity important? What color is desired? Is reversibility important? Are rubbing qualities important? Is flexibility important? Is health and safety a concern? Each of these questions is addressed in more detail below. Q. What is the intended use? Figuring out the best finish for a wood project begins with a clear understanding of how the item will be used and the properties that are most important for that use. . If the item will be used outdoors, a long oil finish such as spar varnish will hold up better than other finishes. . For kitchen tables, you'll want a film-building finish with good resistance to scratches, water, and solvents. Viable candidates here include polyurethane varnish, conversion, water-base, and maybe lacquer. . If you're refinishing an antique, shellac is typically the finish of choice. . For utensils and other items that will be in contact with food, shellac is also a good choice (though an argument can be made that any cured finish is food safe). . For interior floors where scratch resistance is important, polyurethane varnish or water-based finishes are good choices. . If the item will primarily be a display piece that is not subject to wear and tear, it's hard to beat the look of a hand-rubbed oil finish. . For laboratory benches and other institutional furniture, a conversion finish such as catalyzed lacquer is an excellent choice. Q. How durable and protective does the finish need to be? The intended use of the furniture item will go a long way towards determining the level of protection and durability required of the finish. In any event, you need to be able to answer this question. If a high degree of scratch resistance, heat resistance, and water resistance is required, you can summarily dismiss wax, pure oil finishes, and shellac and focus on oil and water-based varnishes and conversion finishes. They do a much better job of protecting the wood. Q. How will you apply the finish? Wood finishing expert Bob Flexner considers access to spray equipment to be the most important consideration in choosing a finish. If you do, he recommends using shellac, lacquer, conversion, or water-based finish. If you don't have spray equipment, oil finishes, oil-varnish blends (e.g., Watco), and wiping varnish are easiest to apply since they can all be wiped on with a rag. However, none of these wipe-on finishes are exceptionally protective so you have to balance ease of application against level of protection. This is a prime example of a wood finish trade-off. Q. What is your skill level? This question ties in with the previous one. If you are relatively inexperienced at finishing wood, you may want to stick with the wipe-on finishes for your initial projects -- wax, oil, oil/varnish blend, and wiping varnish. If you're a more experienced finisher, brushable finishes such as shellac, polyurethane, and brushing lacquer are viable options. If you're thinking of graduating to spraying, just keep in mind that it will take time to develop proficiency with spraying in order to get decent results (i.e., an expensive cherry armoire may not be the best choice for your initial foray into spraying). Q. Do you want a natural, close-to-the-wood look? This is basically a thin versus thick film question. If you want the natural wood look, this can be achieved with most any finish if it is applied thinly. In practice, pure oil, oil/varnish blends, or even plain old wax are most commonly used due to their ease of application. Wiping varnish also works well and provides a bit more protection. If you're looking for a thicker finish that accentuates depth and fills the wood pores, varnish, shellac and lacquer are best. Such a finish is often used on table tops to create a smoothed and polished surface effect. Q. Is clarity important? To get the appearance of greatest depth, go with a high transparency finish such as dewaxed shellac, lacquer, alkyd varnish, or phenolic varnish. The least clear finishes are conversion, natural shellac, water base, and polyurethane varnish. Q. What color is desired? All "clear" wood finishes impart some degree of color to the wood -- and this can be a good thing. For example, oil-based finishes such as varnish work well on darker woods because their yellowish color lends a rich warmth to the wood. They will also yellow more with age. For a more pronounced amber/orange color, there is shellac. On the other hand, these colors may be objectionable on light-colored woods such as holly or maple or woods with a white stain applied. For these situations, blonde shellac, acrylic lacquer, or water-based finish are better choices. Q. Is reversibility important? If ease of repair and removal is important, consider using shellac, lacquer, or pure oil finishes. Some rules of thumb: 1) finishes that are the most easily repaired or removed are also the least solvent resistant; and 2) the easier a finish is to apply, the easier it is to repair (generally speaking). Polyurethane varnish and conversion finish are examples of finishes that are difficult to repair. If you sand them too aggressively to repair a scratch, you may cut through one or more layers, leaving behind witness lines. And since these finishes do not fuse together the way lacquer and shellac do, the witness lines won't go away just by adding more finish. You've got yourself a bit of a problem at this point... Q. Are rubbing qualities important? Rubbing out a finish is done to remove any minor imperfections in the finish such as dust nibs or brush marks and to achieve a desired level of sheen from satin to glossy. It can turn a good finish into a great finish. How well a finish rubs out depends on the hardness of the finish. The hardest finishes - such as shellac and lacquer - rub best. Varnish, water based and conversion finishes are more difficult to rub; they are tough but not brittle hard. The best rubbing finishes also fuse together well when subsequent coats are applied (the fusing prevents witness lines from forming). Shellac and lacquer fuse best. Water base and conversion finish fuse fairly well. Polyurethane varnish does not fuse at all so rubbing out can be problematic if you rub through the top layer. Q. Is flexibility important? Flexibility refers to the pliability of the finish. It is the opposite of brittleness. A flexible finish is appropriate for decks and other outdoor items subject to moisture and temperature extremes because it is able to "give" without cracking or separating. Flexible finishes have a higher oil to resin ratio than other oil-based finishes which makes them not only pliable but also relatively soft (they do not rub out as well as harder finishes). An example of a "long oil" finish is spar varnish, which is often used for boats and related marine applications. Interior woodwork varnishes are typically short to medium-oil varnishes because hardness is generally more important than flexibility. Q. Is health and safety a concern? The solvents in wood finishes release fumes that are toxic, flammable, and air-polluting. Thus, they pose both personal and environmental safety risks. Finishes with the least adverse health and safety effects are water base and shellac. Water base finishes use very little solvent compared to lacquer, shellac, and varnish. Shellac uses denatured alcohol which is less air-polluting than petroleum-based solvents and relatively safe as long as you don't drink it or breathe in too much of it. With any finish, make sure you have adequate ventilation and try to dispose of your waste material in an environmentally responsible manner.
