Outdoor Finishing | Popular Woodworking

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When I was a kid, my dad made a pair of Adirondack chairs that sat out front of the old farmhouse I grew up in. Looking back, they added a quaintness to the house. They were attractive pieces, but in time the finish started to peel off. As other projects came up, refinishing the chairs was put on the backburner. Years went on and they slowly became gray and cracked. Those chairs still sit there, just outside the door, battered, beaten, and colorless.

Naturally, I’ve been haunted by those chairs since. I dread watching a piece of furniture that I put so much time and effort into making drain of color and waste away. Lucky for me, there’s a few options when it comes to finishing for the outdoors, all of which are capable of keeping my pieces safe and my mind free from worry.

First of all, what separates an outdoor finish from a standard indoor finish? The main difference is the additives that, while unnecessary for an indoor piece, will save the life of your outdoor furniture. As you’ve probably seen before (and I in my youthful horror witnessed) raw wood left out unprotected will become a rough, gray mess from the elements. The big three forces that take a toll on outdoor furniture are fungi, moisture, and ultra-violet (UV) light damage. While we can’t completely avoid all of these things, we can give ourselves a fighting chance with some careful planning. 

All outdoor finishes have some means of keeping humidity, rain, and any other sort of moisture from the wood. Spending all day in the sun will really take its toll (as anyone who burns as easily as me can tell you), which is where UV inhibitors come into play, helping to preserve the wood’s color. Fungi like mold and mildew will eat the wood away if left alone, so mildewcides are mixed in to keep pieces from rotting.

Now for the kinds of protection outdoor finishes offer. When you break it down, there’s three basic kinds of outdoor finish: film-forming finishes, penetrating oils, and good old paint. Exterior grade paint has the necessary additives and provides a thick barrier over the wood. Film-forming finishes also create a layer of protection, though theirs is often thinner than that of paint, as well as being transparent or semi-transparent. Finally, penetrating oils soak into the wood itself to seal it and help strengthen it, though they do need to be reapplied more often.

Cedar, redwood, white oak, and cypress are all excellent choices for building outdoor furniture. Whether it’s the resins, the oils, or the way the wood grows, each of these features a way to resist moisture, rot, and bugs.

A Good Starting Place

Before diving into the options for outdoor finishes, I feel it’s necessary to first talk about building the furniture itself. One of the first things to consider when building outdoor furniture is material selection. While aesthetics is usually all I consider for an indoor project, a project that’s going to live outdoors merits a little more thought than that. Domestic woods like white oak, cedar, redwood, and cyprus are all naturally resistant to moisture and rot. More exotic hardwoods like ipe and teak are good choices too for the same reasons. Now, I’m not implying that these woods are entirely waterproof or immune to rot. Given enough time and moisture exposure, they’ll break down just the same, but the dense fibers and natural oils of these woods will keep moisture, fungi, and insects at bay the longest, and when it comes to outdoor furniture I’ll take all the help I can get. 

Once you’ve chosen your wood, it’s wise to give some thought to the adhesive as well. In my opinion, standard wood glues won’t cut it. While they are somewhat water resistant, a true waterproof glue will work much better after years outside. Applied much like standard glues, waterproof glue is specifically designed for exterior use. 

Waterproof glue, two-part epoxy, and polyurethane glue are all good choices for outdoor adhesive. They’re strong, resilient to the elements, and, best of all, waterproof.

Two-part epoxy also makes an excellent outdoor adhesive. Epoxy’s slow set time comes in handy for complex glueups, such as a trellis. 

Polyurethane glue also deserves some time in the light here. I grew up using polyurethane to seal most projects, and exterior polyurethane can be a great choice of film-forming finish, but it took me some time to appreciate its use as an outdoor glue. It does foam as it dries, which may turn some off of it, but it provides a strong hold that’s impervious to water. 

As one last note on building, if your project has hardware (such as bolts or screws) be sure you’re picking up stainless steel or coated hardware. All your work on the finish will be for naught if the project gets stained from a bolt that’s starting to corrode.

Good Old Paint

First on our list is the classic choice of painting the wood. There’s a reason so many things are painted, and it’s not just for the fun colors. A few layers of paint can provide protection to anything it’ll stick to. I consider paint the most practical choice of finish on this list, providing a durable protective coat over all the wood. In exterior-grade paint, additives help stop UV rays and fungus, plus it’s available in the entire rainbow’s worth of colors.

Applying paint is a simple enough task as well. I’m sure most of you out there have painted something at least once in your life, but exterior paint has gotten even easier to apply in recent years, as many paints come with the primer mixed in already. Simply slap a few coats on and you’re good to go.

Of course, the drawback to paint is hiding the natural beauty of the wood. While in some cases this could be good, and paint may provide the best outright protection, I can’t always bring myself to cover up the grain. In those cases, I look to the other two types of finish.

Film-Forming Finishes

Finishes such as spar varnish, exterior polyurethane, and deck sealer all fall into the film-forming finish category. These create a thin coat over the wood to protect it, allowing you to enjoy the grain of the wood while still granting protection from the elements. The finishes are similar to the varnish and polyurethane used indoors, but they’re often mixed with more solids for a thicker coat, along with UV inhibitors and mildewcides for added protection. 

Film-forming finishes are applied similar to indoor varnishes or polyurethanes.

To apply, brush an even coat onto the sanded piece and let it dry. Apply several coats this way, sanding lightly in between. Most film-forming finishes will last you for three to five years, depending on how much punishment nature decides to dish out during that time. Eventually you may notice parts of the finish beginning to wear down or peel off. This is where a film-forming finish gets a little tedious, as all of the finish must be sanded off before refinishing the piece. While this finishing process can get tiresome, a film-forming finish is still an excellent choice of protection when you want the grain to remain.

One larger surfaces, rollers can be used to apply penetrating oils.

While film-forming finishes are available in clear versions, they don’t protect as well against the sunlight as a pigmented option. Because pigment reflects certain light, it offers some additional protection, and a bit of color can really make different woods pop. For instance, the ruddy color of cedar is complimented well by red finish. In general, the darker the color, the more protection offered against UV.

Outdoor finishes, such as these from ZAR, included UV inhibitors to keep your project looking good.

One final consideration regarding film-forming finishes is that clear or not, the finished piece will have a glossy look. This may be great for some pieces (and maybe less so on others), but it is an important thing to keep in mind before committing to a finish.

Penetrating Oils

The third option for outdoor finishing is penetrating oil. Unlike the other options here, penetrating oils (as the name implies) seep into the wood in order to protect it. While this does mean the wood itself is on the front lines, the UV inhibitors, water repellents, and mildewcides mixed in will keep your wood from graying or rotting.

Similar to indoor stains, penetrating oils have a selection of pigmented options. Opaque oils last the longest has they have the most pigment, but they obscure most of the grain.

Applying a penetrating oil is similar to applying an indoor stain or a standard finishing oil. Once the piece is sanded and clean, use a brush, rag, or roller to apply the oil and let it soak for about fifteen minutes, wiping off the excess afterwards. Work with the grain where possible to get an even coat, and be sure to work the oil up into any corners. I usually apply another coat after it’s dry, but there’s no use in applying more than three coats, since the oil won’t build up a barrier.

The issue with penetrating oil finishes is that they’ll be absorbed by the wood before paint or film-forming finishes would start to peel, leaving the surface unprotected. On average, this means you’ll need to reapply the finish every year. 

However, this point also leads me to one of my favorite aspects of penetrating oil: the ease of refinishing. Although penetrating oil needs to be reapplied more often than a film-finish or paint, you only need to clean the wood and reapply the oil. This makes it a great choice for larger outdoor pieces that are difficult or impossible to move inside for refinishing.

As with film finishes, penetrating oils come in a variety of colors, and more pronounced color will give better UV protection.

Epoxy

One last note should be made for epoxy. There’s a trick to making paint and varnish more durable that we can steal from shipwrights: applying epoxy to the wood before putting on the top coat. For these purposes, you’ll need an “epoxy coating” rather than one that’s specifically an adhesive. 

To apply, mix as the product directs you to and brush it onto the wood. As it cures, the epoxy seeps into the wood fibers and hardens them, making the surface more durable. Once cured, you can apply your paint or varnish.

Applying an epoxy “shoe” to the foot of piece seals the pores of the end grain and forms as protective pad. Not only does this help prevent wear, ut it also closes the ends of the fibers to prevent “wicking” of moisture up the leg of the furniture piece.

Another use for epoxy is making a “shoe” for your outdoor furniture. End grain sucks up moisture readily, and it’s the most easily worn place on any outdoor piece. By making a reservoir of tape around the foot and applying a thick layer of epoxy, it seals the end grains and creates a waterproof shoe that protects those end grains from wear. Epoxy can also be used to seal up any cracks or knots before finishing. Just pour a little epoxy in, wait for it to dry, and sand it down.

The issue with epoxy is that the cost can add up quick. However, if you’re looking to ensure that a project will stand the test of time, then some epoxy may just be worth the investment. 

Aging Naturally

As much as I’ve talked about keeping your outdoor furniture safe from the elements, there can also be beauty in the way it weathers. In some cases, you may want a piece of outdoor furniture to have an aged look. In this case you may be better off with no finish at all. 

Some woods (such as cedar, redwood, ipe, white oak, or teak) can develop an intriguing, rustic look as they age. These woods grow into their gray well, becoming reminiscent of old barns and farmhouses. This distinctive, antique aesthetic can’t be duplicated with anything shy of a professional paint job. The weathered look of natural aging isn’t going to work with every project, but sometimes the nostalgic tone of grayed wood can be a nice change of pace. 

Though it will wear eventually, a well-built piece can last for some time unprotected. The Adirondack chairs from my childhood still sit outside that house decades later. They may not be what they once were, but maybe I was too hard on those chairs. Maybe a little change can be a good thing.


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