Oil vs. Film, Choose Your Finish
For many woodworkers, the contest to choose a finish ends in round one. Whether it's polyurethane, Danish oil, or spray lacquer, the winner never changes. But the finish you like best may not be the best one for your project, depending on its style and use.
All types of finishes have strengths and weaknesses, and a strength in one situation may become a weakness in another. For example, do you want a finish that's thin enough to apply fast and easily, or one that builds a thick, protective film in just a couple of coats? Should it be water- and chemicalresistant or easy to repair? Do you want a surface you can rub out to an even sheen or one that resists abrasion?
Clear finishes fall into six categories, each with its own mix of characteristics.
Drying oils, including boiled linseed oil and pure tung oil, penetrate the wood surface and react with oxygen to cure. Boiled linseed oil contains drying agents that let it cure overnight, but tung oil can take several days. Tung oil, however, darkens less with time. Beware: Some finishes with "tung oil" in their name contain little or no real tung oil as an ingredient.
Oil/varnish mixes, such as Danish oil, enhance grain while laying down a thin film. Because of their oil content, oil/ varnish mixes also need to dry overnight.
Lacquer combines nitrocellulose solids in a mixture of solvents to create a film finish that dries quickly and adds a faint amber tint. For a clearer finish, use cellulose acetate butyrate (CAB) lacquer. Unless you buy a formula that dries slowly enough to brush on, plan to spray most lacquers.
Water-based finish goes by many names that may include the words "lacquer" or "polyurethane," but most use acrylic or acrylic/polyurethane resins in solvent and water. By eliminating oil, and using acrylic and other clear resins, water-based finishes don't yellow as they age.
Wiping varnish, usually a type of polyurethane, comes premixed, but you can make your own by thinning regular varnish with mineral spirits.
Polyurethane varnish mixes alkyd and polyurethane resins with different types of drying oils. Unlike an oil/varnish blend, however, the ingredients are heated until they combine. Reducing the ratio of oil to resin creates a hard finish for indoor use. Increasing the amount of oil makes the finish flexible enough to withstand wood movement outdoors.
Now that you know the players, learn how to get the most from them.
Below, we've arranged these finish types with durability increasing from beginning to end. As durability increases, though, ease of repairs decreases. For example, lacquer offers little resistance to alcohol or water, but it's as easy to repair as spraying on a fresh coat. Polyurethane varnish ranks tops in moisture and chemical resistance, but repairing deep scratches requires stripping the old finish.
Easy to apply: Forget about streaks, drips, and brush marks. Just wipe on boiled linseed oil with a soft cloth or flood it on until you've saturated the wood surface. Then wipe off the excess and let it dry overnight before recoating, or one week before applying a film finish. To avoid spontaneous combustion from finish in the oil-soaked rags reacting with oxygen, hang or spread them out to dry before discarding.
Easily damaged, easily repaired: Drying oil cures too soft and thin to protect against moisture vapor or abrasion. It also darkens with time as it continues to oxidize. But the lack of a film build makes this the easiest finish to repair: Simply apply another coat of finish to damaged areas or when the surface starts to show wear.
Success secrets: Reapply oil where it soaks in completely, but wipe away surplus surface oil or it will cure soft and gummy. If the oil warms after it goes on, watch for "bleeding" from the wood pores, and wipe away these droplets before the oil cures. This may need to be done more than once, so check it periodically. Should you later want a more durable finish, oils still highlight grain beneath a film finish.
Try it on carvings and objects you'll handle infrequently where you want a natural, unfinished look. Also, apply it beneath a film finish to add an amber tone and to accent figured woods such as curly maple.
But avoid it for any project that requires moisture or abrasion resistance, especially outdoor projects. Oil provides almost no surface protection and does little to block moisture vapor, making it a poor choice for most furnishings you'll use every day.
Goes on with a cloth: Comparable to drying oils for ease of application, these mixtures also penetrate the wood. The difference? They leave an extremely thin, soft surface film. As with oil, soak the surface evenly before you wipe away the excess. When discarding rags, follow the same cautions as for drying oils.
Minimal protection, but easy to fix: Adding varnish to these mixtures doesn't significantly increase a project's abrasion, moisture, or heat resistance compared with boiled linseed oil. But the very thin build makes oil/varnish blends easy to repair or retouch. Just smooth over light damage with 280-grit sandpaper and apply a fresh coat.
Success secrets: Drying oils and varnishes are compatible with each other when mixed, so you can create your own blend by combining equal parts of boiled linseed oil and any oil-based varnish. Increase the percentage of varnish to increase gloss, hardness, and moisture resistance. As with oil, flood the surface thoroughly and recoat places where the mixture soaks in completely. You even can use oil/varnish blend as a lubricant while finish-sanding with 600-grit or finer sandpaper, wiping away the surplus.
Try it on indoor projects wehre you'll want a soft, satin sheen, or a simple, rustic finish as on a simulated antique. Because it's easy to repair, you also can use it on shop-made workbenches and on tools such as the deadblow hammer shown below.
But avoid it for surfaces you'd rather protect against wear and abrasion instead of constantly repairing the finish, as on dining tables or chairs.
Easy to spray, harder to brush: Whether sprayed from a gun or an aerosol can, lacquer lays down a smooth, dust-free finish that dries quickly. However, high humidity can turn spray lacquer opaque, called "blushing." Slower-drying "brushing" lacquers provide another, though trickier, way to apply lacquer. Both types require a well-ventilated finishing area and lacquer thinner for cleanup.
Repairability/durability: Even old coats of lacquer can be dissolved with lacquer thinner for easy stripping. Or simply spray on a fresh coat to partially dissolve the uppermost layer and cover minor scuffs and scratches without refinishing. Lacquer dries to a hard surface that resists abrasion, moisture, and solvents less than varnish.
Success secrets: Customize lacquer to work in a variety of spray setups and climates by adding lacquer thinner to control viscosity and retarders to slow the drying speed. Don't have a spray gun? Aerosols work just as well, although they're thinned so much that more applications are needed. Apply spray-on lacquer in low humidity to avoid blushing. Lacquer's hardness makes it easier than polyurethane varnish to sand and then rub out the finish to a high shine. Avoid contaminating the wood surface with silicone—found in some lubricants and furniture polishes—which can produce small dimples in the surface, called fish eye.
Try it on all furnishings not subject to moisture contact or rough handling. Use CAB acrylic lacquer as a crystal-clear alternative to water-based finishes for a non-yellowing film.
But avoid it for high-abuse projects such as kids' toys and furnishings.
Quick drying, quick cleanup: Water-based finish sprays or brushes on easily, but it dries quickly enough that you have to work fast to avoid brush marks. Water-based finishes release fewer odors than oil-based varnish or lacquer, but still contain solvents. Apply them in well ventilated locations while wearing a respirator. Soap and water take care of cleanup before the finish dries.
Moderately durable, but a problem to patch: Water-based finishes won't redissolve like lacquer or cure thin like an oil/varnish mix, making them harder to repair than lacquer or drying oils. Waterbased finishes compare to lacquer for durability, but deteriorate from chemicals such as glass cleaners with ammonia, and from constant contact with bare skin.
Success secrets: To minimize grainraising problems with water-based finishes, first moisten the grain to raise it and gently sand away nibs before applying a sealer coat. Water-based finishes dry slowly enough to form runs, so apply light coats and sand lightly between coats with 220-grit abrasive. Avoid rebrushing freshly applied finish, which can leave streaks and bubbles. Also, apply water-based finish when the temperature is 60—90° F and the humidity is 50 percent or less.
Try it on projects where you'd use oilbased varnish but don't require its abrasion and moisture resistance. The clarity of water-based finishes makes them ideal for light woods such as maple, where you want to preserve the wood's natural color.
But avoid it for projects where you need exceptional water and chemical resistance.
Wipes on fast, but builds slowly: Almost as easy to apply as an oil/varnish blend, wiping varnish dries quickly enough to reduce brush marks and dust nibs. You also eliminate runs and sags when applying wipe-on finishes in thin coats. Using a rag instead of a brush eliminates cleanup. The downside: You'll need to apply at least two coats of wipe-on varnish to equal one coat of full strenght varnish.
More coats equal more durability: Although each coat of wipe-on varnish goes on thin, the film it leaves complicates repairs as much as unthinned varnish. Sand and recoat light scuffs and scratches. For deep scratches—especially on stained wood—strip and refinish the surface. Allow finishsoaked rags to dry before discarding them.
Success secrets: Avoid the temptation to apply thick coats, which can run and drip. Premixed wiping varnishes save time, but you'll save money by making your own. Thin full-strength varnish 25-50 percent with mineral spirits until you achieve a balance of smoothness and thickness. You can use polyurethane, but an alkyd-resin varnish also will work. Despite the name, you can brush on wiping varnish, although that works best on horizontal surfaces, where it won't run or drip.
Try it on any project where you would use full-strength varnish but have the luxury of time to apply several coats. It's especially useful for reaching into nooks and crannies on carvings or routed profiles.
But avoid it for surfaces that must be easy to repair. Like full-strength varnish, it will also add an amber or yellow cast to light woods, including pine and maple.
Brush with care: Slowdrying full-strength polyurethane has a frustrating knack for catching dust nibs and, when applied too thickly, running and dripping. It's better brushed than sprayed because overspray sticks to everything. Polyurethane's abrasion resistance makes it difficult to rub out, and cleanup requires using mineral spirits.
Tricky to apply, but difficult to damage: Nothing short of chemical strippers or aggressive sanding will remove polyurethane, but that also helps it survive wear, moisture, and chemical damage in the first place. Plus, polyurethane's heat resistance makes it a sound choice for projects such as this casserole carrier shown below.
Success secrets: Despite what it says on the label, polyurethane can be thinned 5 - 10 percent with mineral spirits until it brushes on smoothly and dries slowly enough for bubbles to pop and brush marks to level out. More so than with any other finish shown here, success depends on applying poly to a clean surface in a dust-free environment.
Try it on any project where you want high wear resistance. Choose it for projects that will take the most daily abuse such as kitchen tables, chairs, and kids' toys.
But avoid it for furnishings you'll rub out to a high-gloss shine, or projects where easy repairs are essential. It's also less convenient to apply than spray lacquer or even wiping varnish for display pieces such as a mantel clock or decorative box.
By Bob Wilson with Bob Flexner
© Copyright Meredith Corporation 2006