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. Whether it's a varnish for outdoor wood or an epoxy finish, this guide will help you find the right finish for your project.
No outdoor polyurethane varnish or paint will completely protect your projects from the rain, snow, and sun. Nevertheless, choosing the best outdoor paint for wood or knowing how to oil outdoor furniture is critical to keep 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, those finishes that offer better protection at the beginning can also be more difficult to maintain and refinish.
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.
Oil-based Varnishes for Outdoor Wood
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 outdoor polyurethane 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. Nevertheless, polyurethane varnish has many benefits and uses. Explore a detailed guide to various wood varnishes from Atlanta Specialty Woods.
How to Oil Natural Exterior Wood
Depending on the weather, a well-applied varnish for outdoor wood 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.
To learn how to spray, read this free article: Spraying Made Simple.
Water-based Varnishes for Outdoor Wood
These film-forming finishes, such as General Finishes Exterior 450, 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, these clear outdoor finishes dry 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.
How to Apply Water-Based Varnish
Water-based outdoor polyurethane varnish 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 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.
Sealers: Keeping Water Out
While they are not a complete finish, sealers, like Seal-Once Nano Guard, enjoy a symbiotic relationship with film-forming finishes such as varnish for outdoor wood, enhancing their protection. System Three S1 Epoxy is also a sealer and consists of two parts—a resin and a hardener. Mixing them creates a thin epoxy finish with an open time sufficient to allow brushing or spraying. The pros and cons of epoxy coating for outdoor surfaces are worth understanding. Other sealers are single-part liquids. Both options soak deeply into the wood with the same goal: preventing water penetration and therefore rot.
Do Sealers Protect Against UV Rays?
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, like epoxy finish, locally, where they provide the most benefit, such as on the end-grain feet of outdoor furniture. Here are a few epoxy tips for outdoor use.
Finishes for Small Projects
While large outdoor projects that must endure the elements deserve a finish, you may wonder how to oil natural exterior wood on smaller objects such as tool handles. 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.
How to Finish Small Outdoor Projects
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.
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.
Learn how to create a milk paint masterpiece.
What is the Best Outdoor Paint for Wood?
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 for outdoor wood over milk paint will help maintain its color, enhance UV protection, and add some gloss.
For more information, learn about the best paints to use on decks and exterior wood furniture.
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 a 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 vs Water-based Stains
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.
Penetrating Oils: Working From Within
Unlike film finishes such as outdoor polyurethane varnish and paints, which form a barrier on the wood’s surface, penetrating oils, like Osmo, 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.
How to Apply Penetrating Oils to Exterior Wood Projects
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. As for how to oil natural exterior wood, 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.
Can shellac be used outdoors?
While shellac is a beautiful, fast-drying finish for interior work, it’s a poor choice outdoors. It has little resistance to moisture, heat, or UV exposure, so rain and humidity can cause it to turn cloudy, soften, or fail outright. Sunlight further degrades the film, leading to cracking and peeling. Even brief exposure to the elements can damage it, making shellac impractical for exterior projects where durability and weather resistance are essential.
Other Finishes to Avoid Using Outdoors
In addition to outdoor shellac, other finishes simply aren’t built for exposure to the elements. While they perform well indoors, they break down quickly outside when faced with moisture, UV light, and temperature swings.
- Shellac
- Wax
- Lacquer
- Anything labeled as interior-only finishes
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.
Which Woods are Better, Domestic or Exotics?
Domestic species renowned for their decay resistance include:
- Cedar
- Cypress
- Redwood
- White oak
- Catalpa
Decay resistant exotics include:
- Mahogany
- Teak
- Ipe
Or consider using thermally modified stock, 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.
When working with wood such as sheet goods for structures and construction jobs, you may need a few building hints from APA.