A coworker recently asked me about finishing wood for outdoor use. I’ve gone round and round about this but have come to some conclusions I think are worth sharing. Here are some things that I’ve discovered that have helped me make decisions in this regard.

  1. Painting or staining are cosmetic, they don’t seem to protect the wood that much. I’ve seen bad paint jobs do more harm than good. So it’s not really a big deal to leave wood unfinished to weather, even in mixed climates. We have a fence with unfinished pine rails fixed to steel posts, and the rails are still straight and strong after eight years.
  2. Anything that leaves a film (that is, paint or urethane) will eventually crack and peel, allowing moisture to enter. If it isn’t refinished it will trap moisture in the wood, causing rot or other damage. So using paint or a urethane is like making a commitment to refinish for the remainder of the project’s life. Filmy urethanes on decks tend to get cheesed underfoot, too.
  3. Penetrating sealers degrade more quickly than paint or urethane, but they degrade much more gracefully (they don’t peel) and they can be more or less safely neglected. They won’t get cheesed underfoot either.
  4. If left to weather, pine is known for being dimensionally unstable. That is, it will eventually warp or crack. Pressure-treated is particularly notorious for this. Redwood on the other hand is more rot-resistant, it is very dimensionally stable, and it ages gracefully… it’s more expensive up front but I don’t have to worry much about keeping up after it. So I think it pays for itself in the long run. (I’m in the Southwest so redwood is more available than, say, cedar.)
  5. Of course, keep all wood away from ground contact to avoid termite damage and rot.

I hear good things about handmade oil-based stains made from raw linseed oil and citrus thinner. I’ve seen beautiful results. It is a little more work than just buying a gallon of whatever at Ace, but there are some real benefits. Namely that the result is deeply personal (it is whatever color you mix), it doesn’t hurt the wood if you neglect to refinish it, and it can be totally non-toxic and biodegradeable (so you don’t have to dispose of the excess with hazardous waste). The ingredients can be hard to come by, though, especially citrus thinner and pigments.

We haven’t tried the linseed oil-based stain yet but we’d like to do some experiments on our Potty House sometime. It is made almost entirely out of salvaged wood, much of which had been painted at one time. Most of the old paint sanded right off but there are still some pieces where the paint held on; I’m curious what will happen when we try using a 50-50 mix of linseed oil and citrus thinner over the old paint.

Update 1/6/2010

Lately I’ve been experimenting with a no-VOC stain, specifically BioShield AquaResin #33 in Teak. So far I find it a joy to work with and it produces a beautiful finish that seems durable, hides defects, and yet does not hide the wood grain too much. I used it to finish the top of a wine rack and am experimenting with using it for architectural interior trim and also for an exterior wood door. I’m pleased so far but we’ll see how well it holds up on the wood door. Though the door is well shaded and protected with an overhang, it faces west and gets some harsh sun in the summer.

The nice thing about the stain is that surface prep is minimal, it’s easy to apply, it has virtually no odor, and clean-up is no fuss. So unless the surface peels (it shouldn’t) it won’t be much trouble to refinish the door every couple years.