Sealing exterior Douglas Fir post and beam structure

[scott vroom]

We just finished building a gable roof over our patio and are considering how to finish the doug fir open posts/beams/rafter ties. I think stain is going to look blotchy, was thinking a clear oil base varnish to bring out the grain would look best. Anyone have experience with this? Appreciate any and all inputs.

[Richard Coers]

Any clear finish on an exterior structure will become a part time job like a wooden boat does. A film finish will really require the effort. Sunlight will degrade the finish, and wood will move. That means a crack in the finish and water will get behind the finish and it will start to flake off. To fix that you will have to sand it and refinish. Probably at least every 4-5 years. Wait too long and the wood will change color where the damage is. I'd use an oil or sealer if you really have to put on a finish. You'll still be on about the same schedule, maybe a slightly shorter schedule at first, but you won't have to do the sanding.

[Paul F Franklin]

If you are set on a clear film finish, get the best marine grade spar varnish you can afford. It will have UV inhibitors, and is generally formulated to be a little more flexible when dry to help accommodate wood movement better than lesser finishes.

But Richard speaks truth. All outdoor finishes (as opposed to no finish and natural weathering) require maintenance, and clear more than others.

[Randall J Cox]

Agree with Richard and Paul. I have used Superdeck stain with great results over the past 10 years on the wood siding on my cabin and its decking. Its at 5200 ft in elev and the sun is hot up there. Its not cheap, probably $40 per gallon but a very good oil stain. I use semi transparent for the max protection. They have a natural. Lets the grain show much better but with less uv protection. Randy

[Edward Dyas]

Douglas fir will be high maintenance regardless of what you use. You could use an oil finish but it would need to be re-treated every 3-4 months for the first couple years and then annually. You could finish it with a marine grade spar varnish and the duty life would vary depending on the quality of the varnish. The best is Epifanes. Depending on how much sun it gets you could probably get six years before it shows signs of needing another coat. A varnish will start looking dead and dry. When it does this it get porous and allows water to pass through it. Then the water gets under it and starts it to lift. Most people ignore the finish until it gets to this point and then they have a big job to chemically strip the finish and start over. If they had only put a fresh coat on before water started getting through the finish it would have lasted much much longer.

[scott vroom]

Thanks for the inputs. Looks like film build-up sealers are not what I want since sanding down the road is out of the question. That leaves either an oil base penetrating sealer like Penofin, Sikkens, or maybe Superdeck as recommended above. Or paint.

[Tom Hyde]

I've used Sikkens products on exterior Douglas Fir. Did pretty well for a while. Actually, in areas that never get direct sun (front porch), it still looks like new 15 years later. In areas with direct sun, it needed refinishing within 5 years.

You might want to do a little research by looking at what timber frame companies use. I know Heritage Natural Finishes (nontoxic) are popular within the industry but I've never used them myself.

Edit: Another option, which I hesitate to mention, is WoodRX which is available at big box stores. It will hide quite a bit of grain, it has silicon in it which many people hate, and in the "natural" or "cedar" color tends to impart an orangish spray tan look I don't like (kinda hate, actually). The "teak" and darker colors aren't bad, though, but hide even more grain and that beautiful rich natural color of Doug Fir. BUT, if you are looking for an exterior finish that will hold up for as much as 10 years or more, I think it's hard to beat. I know a premium custom cedar gate maker that recommends it, and only it, as an exterior finish (though prefers to let the cedar age into a gray patina naturally).