Forest Products Laboratory Witch’s Brew

June 2, 2013 § 4 Comments

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One of my long, long overdue projects is a fence for our garden beds. I’m still chipping away at it, but at least I’ve officially entered the – “Oh, yeah, it’s almost done” when asked – phase of the project. I made the fence and gate from Western red cedar, a wood that has almost no peers when holding up to the weather year after year. I wanted a natural look to the fence – not painted – but most outdoor finishes don’t last too long. The combination of UV rays, rain, and snow work right through most clear outdoor finishes in just a few years, as anyone with a deck can readily attest. To save the constant maintenance, many opt to let cedar naturally age a silvery gray. And others buy pressure treated wood, which has similar resistance to decay, especially when in contact with the ground.

Not being a huge fan of pressure treated wood, I wanted to use cedar for this little fence and planned on just letting it age naturally. Then I read something in Christian Becksvoort’s With the Grain about an outdoor finish that rivaled pressure treated wood in durability. A 1984 study by the Forest Products Laboratory showed that a simple mixture of spar varnish, paraffin wax, and thinner (mineral spirits or turpentine) creates a highly effective wood preservative. For a gallon batch, the recipe calls for 3 cups of spar varnish, 1 ounce of wax, and then enough thinner to fill out the gallon. The logic makes sense. Wax is an excellent water repellent, and the thinner acts to drive the wax into the pores of the wood. Spar varnish is added to help with UV protection. Having all the ingredients already on hand, I figured what the hell.

I adjusted the ratios down to a quart size, and used bees and carnauba wax instead of paraffin. I also added more wax to my concoction. The Forest Products Laboratory study called for painting the surface after the preservative dried. That probably explains the relatively small amount of wax. Any more wax and paint would have a hard time adhering to the wood. Because I wasn’t painting the fence, I added more wax. I melted the wax in a double boiler, added the spar varnish, and then transferred the two into a quart can. I then poured enough odorless mineral spirits into the can to fill the quart. When applied (I just slapped two coats on with a brush), the finish gives the wood a nice wet look similar to most finishes, but when it dries it’s completely different. It’s a little hard to explain, and to show in pictures, but the dried finish gives the wood a completely matte, almost chalky look. It’s not a bad look, just different. The “chalky” look is probably the wax being seen in the pores of the wood, and it’s especially noticeable in the end grain.

We’ll see how will it works. I suspect the positive effects of the spar varnish won’t last too terribly long – UV rays will always win out in the end. But I have hopes for the wax. If it buys a few extra years of life for the wood I won’t complain.

More:
The Forest Products Laboratory website, part of the USDA Forest Service, has a lot of interesting information, including lots of studies like this one pertaining to wood and lumber.

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