Outdoor Finish

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This article is from Issue 101 of Woodcraft Magazine.

Dress your outdoor woodwork for success

When it rains, it pours. So you put on a raincoat—or go inside. And when the snows come, you bundle up. Sunny? A pair of sunglasses and maybe some SPF lotion. But your outdoor furniture and other woodwork don’t have such luxuries. Those pieces rely on you to store them inside or undercover, and when that’s not practical, to apply and maintain a protective finish.

While no product will completely protect your projects from the rain, snow, and sun, choosing the right finish and applying it the right way is a critical opening gambit in the uphill battle of keeping your woodwork lasting long and looking beautiful. After all, the finish has to balance protection from the elements with an element of simplicity in both application and renewal. As you’ll see in the following pages, those finishes that offer better protection at the beginning can also be more difficult to maintain and refinish.

And maintaining an outdoor finish may be necessary, but it doesn’t have to be evil. The trick is picking the right finish, then knowing how and—just as importantly—how often to maintain that finish to protect your projects and keep your wood looking good season after season.

Multi-layered protection. For maximum protection, some manufacturers recommend applying 7-8 coats of their exterior varnishes. Thin as directed and allow to dry before scuff sanding between coats. Clean up with mineral spirits.

Oil-based varnishes

Varnish has come to be a catch-all term for any number of film-forming finishes. Most oil-based “varnishes” consist of a drying oil (usually tung, or linseed), a resin (typically alkyd, or polyurethane) and a solvent (mineral spirits). Varying the type and ratio of those components affects characteristics of the protective layer formed by the film. Exterior oil-based varnishes—some manufacturers dub them “spar” from the days of tall-masted ships—contain a greater percentage of oils than their interior counterparts. This formulation means exterior varnish remains more flexible when cured, lessening the likelihood of it cracking or peeling from seasonal wood movement. Frequently, these finishes also contain UV blockers that inhibit the sun’s ultraviolet rays from degrading the wood beneath. The tradeoff is that exterior varnishes aren’t as hard or durable as interior formulations. Depending on the weather, a well-applied exterior varnish will last 2-3 years before it begins to look chalky. When it does, sand the surface and apply a fresh coat. Do this before the finish develops cracks; otherwise you may have to sand down to bare wood. Oil-based varnishes, which impart an amber hue to the wood, can be applied by brush or spray. When applying to bare wood, it’s often wise to thin the initial coat with mineral spirits (up to 50% with some products) to promote good adhesion.

Toughness on top. A water-based varnish adds protection and gloss to bare wood and painted surfaces. Here it’s being sprayed on a white oak ladder-ball game.

Water-based varnishes

These film-forming finishes are also resin-based (alkyd, urethane, and/or acrylic) but use water as their solvent, making them environmentally friendlier and easier to clean up. Like their oil-based cousins, the outdoor formulations form a weather- and UV-resistant film that remains flexible. Although milky-white as a liquid, they dry clear with no amber cast. This tends to flatten the color and grain rather than enhance it the way an oil-based varnish does. Staining first can help, as can using amber-colored additives made for the purpose. Water-based varnishes can be applied over other non-oil-based finishes such as milk paint to add gloss and durability, but be sure to check for compatibility first. To apply a water-based finish, first raise the wood grain with distilled water (see tip on page 16) and then lightly sand away the raised fibers before brushing or spraying on 3-4 thin coats. Allow each coat to dry thoroughly, then sand and recoat. Clean up with water.

Keep your feet dry. Sealers penetrate wood and fill pores to keep water out, making them particularly useful on the feet of outdoor furniture.

Sealers: keeping water out

While they are not a complete finish, sealers enjoy a symbiotic relationship with film-forming finishes such as varnish, enhancing their protection. Some sealers consist of two parts—a resin and a hardener. Mixing them creates a thin epoxy with an open time sufficient to allow brushing or spraying. Other brands are single-part liquids. Both soak deeply into the wood with the same goal: preventing water penetration and therefore rot. Sealers don’t provide UV protection, and need a topcoat to prevent sun damage and weathering. For optimum protection, seal all project parts (except glue-joint surfaces) with 2-3 coats after cutting the parts to size, but before assembling them. Once your piece is together, topcoat as normal. This added step may well be worth it for significant projects that will be left out in all conditions. Or consider using sealers locally, where they provide the most benefit, such as on the end-grain feet of outdoor furniture. 

Looking sharp. Linseed oil-based gunstock finishes are a great choice for protecting outdoor tool handles. The penetrating oil protects the wood from sunlight, water and grease.

Finishes for small projects

While large outdoor projects that must endure the elements deserve a finish, smaller wooden outdoor-use objects such as tool handles used outdoors warrant protection as well. This calls for something a little different. Gunstock oils—based on linseed oil—are not just for firearms. These drying oils harden in the wood, not on it. This deepens the color and accentuates the grain while providing a durable finish that resists wear and moisture.

GB Lin-Speed Oil can be applied with a lint-free cloth. Spread a thin coat so the wood looks barely wet. Let the finish absorb before adding successive coats—up to 8 or 10 to create a lustrous sheen. Repair or renew the finish as needed by simply rubbing on an additional coat or two.

Brighten up. Add a splash of color to your outdoor projects with milk paint. Whether powdered, or premixed, these coatings offer protection from both sun and weather.

Milk paint: color & protection

Traditional milk paint is a combination of lime (or borax), a pigment, and casein, a protein found in milk. These ingredients make it non-toxic, biodegradable, and fume-free. Milk paint also holds up well outside, and is nearly impossible to remove. Available in powdered form, you mix it with warm water to the consistency of a melted milkshake, then brush or wipe it onto sanded wood surfaces. Thinning it with extra water creates a more rustic “washed” look. The available colors tend toward barn reds, colonial blues, and linen whites. Mix it in small batches, as it doesn’t keep more than a day.

One manufacturer offers premixed “milk” paints in similar, muted colors fortified with added acrylic resins. These water-based paints come in cans and share many of the non-toxic qualities of the traditional powders.

Whether powdered or pre-mixed, milk paint provides a smooth, velvety finish without a sheen. Several coats may be required for complete coverage, which also provides UV protection. Adding a water-based varnish over milk paint will help maintain its color, enhance UV protection, and add some gloss. 

Enhanced protection and color. These sample cedar boards show the spectrum from transparent to solid stain in the same color. Stains are a balancing act, with more opaque stains offering better protection and more transparent stains showing the wood’s inherent beauty.

Stains: perfect for structures

Stains represent some of the most effective and easily applied outdoor finishes around. While generally intended for decking and siding, they are also appropriate for most outdoor projects. Available as water-based, oil-based, or as a hybrid of both, stains come in variety of colors and in three types: transparent, semitransparent, and solid. Transparent stains contain the least amount of pigment, which allows the grain of the wood to show through, but which offers less UV protection. Transparent stains won’t peel, but typically need to be recoated yearly. Semi-transparent stains offer a little more protection and longevity (typically 2-3 years, with no peeling) while still allowing the wood’s grain to show. Solid (or opaque) stains very closely mimic paint, although they do tend to show the grain’s texture. They offer the most protection and longest lifespan (5-7 years) but can peel if the surfaces aren’t properly prepared. Oil-based stains tend to give the wood a more natural look. Water-based stains, with their acrylic resins, are better at resisting UV damage, but can raise the grain. The new hybrid stains combine the characteristics of oil- and water-based products. All of the choices can be applied by brushing, rolling, or spraying. Clean up according to the base solvent. 

Wipe on, soak, wipe off. Thoroughly stir the finish, then wipe it on with a rag, let it soak in per the manufacturer’s direction, and wipe off. A coat or two will do, since penetrating oil does not build a film.

Penetrating oils: working from within

Unlike film finishes such as varnishes and paints, which form a barrier on the wood’s surface, penetrating oils soak into the wood and do their work from within. While there are myriad formulations with names such as “timber,” “teak,” and “outdoor” oil, they all have some similar characteristics. Oils maintain a natural (though darkened) look without adding a glossy coating, so there’s nothing to peel or flake. The flip side is that oils offer less protection than film-forming finishes, and eventually the wood will gray from exposure. Outdoor oils usually incorporate UV-blockers and some contain heat stabilizers and mildew inhibitors. Of all the available outdoor finishes, oil is undoubtedly the easiest to apply: wipe or brush on liberally, recoating every 6-12 months to maintain the protective barrier. Fortunately there’s usually no need to sand before reapplying additional coats.

Selecting the right wood

Regardless of the finish you choose, selecting the right wood is critical to the success of your outdoor projects. Start with a rot-prone species such as sycamore, and your hard work will soon fall apart even if you’re a stickler for maintenance. But choose wisely and you’re likely to see years of service from your projects even with a little benign neglect. Domestic species renowned for their decay resistance include cedar, cypress, redwood, white oak, and catalpa. Exotics include mahogany, teak, and ipe. Or consider using thermally modified stock (see page 52) such as southern yellow pine. 

The usual suspects. Top row, left to right: catalpa, cedar, cypress, white oak. Bottom row, left to right; mahogany, thermally-modified pine, ipe, and redwood. 

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