Exterior oil finishes are one no-peel option for outdoor wood. Though it requires annual re-application, the job is much easier with no previous finish peeling to deal with.  Photo by Steve Maxwell 

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I built an oak mailbox for my suburban home when I was a teenager, and I finished it the same way I finished the indoor furniture I was making at the time: clear satin polyurethane. The mailbox looked good enough that strangers driving by noticed it, came to the door and asked if I’d build one for them, too. Unfortunately, my inexperience with exterior wood finishing caught up with both mailboxes. Within a year, the hand-rubbed finish had peeled and failed badly. I now know better, but the secrets of durable exterior finishing aren’t always obvious. 

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The problem is that exterior wood is a difficult material to protect. It expands and contracts with changes in moisture. It absorbs rain, dries in the sun, freezes in winter and bakes in summer. No outdoor finish lives an easy life. 

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One of the most overlooked issues is wood absorbency. New lumber often looks ready for finishing, but appearances can fool you. Planing at the sawmill leaves a burnished surface that’s too smooth to absorb finishes properly. The surface looks clean and fresh, but it doesn’t “grab” finish as well as it should. 

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This is why sanding matters so much, but not to a super-fine level. For decks, outdoor furniture, benches and most exterior wood, an 80-grit abrasive in a six-inch random orbit sander is ideal. It removes the burnished surface and leaves the wood open enough to absorb finish and hold it. 

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Moisture content also matters. Wood that’s too wet won’t hold a finish properly, even if the surface looks dry. Spring can be especially tricky in Canada because wood may still be carrying moisture from snow, rain and damp weather. A cheap moisture meter is helpful for identifying the ideal 12 to 14 per cent moisture content, but so is patience. Rushing a finish onto damp wood is one of the most reliable ways to disappointment. 

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There’s also an important difference between film-forming and non-film-forming finishes. Paints, solid stains and varnish-like coatings form a layer on top of the wood. These finishes can look excellent and offer strong colour, but they eventually crack, blister or peel when water gets underneath or when wood movement breaks the surface bond. 

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Eventually all film-forming outdoor wood finishes go like this. Sanding back to bare wood is an essential but troublesome restoration step. Photo by Vecteezy 

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No matter how good your prep is, peeling inevitably happens with film-forming finishes outdoors. This matters most on intricate items such as outdoor benches, railings, lattice, chairs and complicated deck structures. Once peeling begins on detailed woodwork, removal and refinishing is necessary but miserable. 

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That’s why non-film-forming finishes deserve more attention. Exterior penetrating oils don’t sit on top of the wood in the same way. They soak in, enrich the surface and wear away gradually rather than peeling. The downside is that most exterior oil finishes need reapplication about once a year. But the upside is big: a quick cleaning and another coat is much easier than scraping and sanding peeling paint. Oil is one of the unsung heroes of outdoor wood finishing.