outdoor finishes???

Re: outdoor finishes???

Don Stephan

Wood is always absorbing or giving off moisture, depending on the relative humidity in the environment and the moisture content of the wood. And no finish is completely impervious to moisture movement. As wood absorbs moisture it increases in width and thickness, as it gives off moisture it decreases in width and thickness. The rate of movement is fairly consistent within each wood but can vary between species. Wood does not change in length due to changes in moisture content.

So for example a 4' square slab of wood a couple inches thick that is surrounded by a tightly fitting steel frame might have problems. A general average rate of movement is 1/8" per foot, so "on average" that 4' square slab should not change materially in length but could vary in width up to 1/2" in width over the course of a year. Some woods move more than the average, some less.

Do your sculptures ever place wood in a situation where it cannot move seasonally? Do you ever combine woods in cross grain situations, for example the end of one slab firmly joined to the side grain of another, where one piece tries to move seasonally and the other does not? Do your sculptures ever combine different species of wood where different rates of seasonal movement will conflict?

In other words, do you need finishes that most successfully resist moisture movement above all other finish characteristics?

Re: outdoor finishes???

Peter Rand

Don - Very instructive answer. Many thanks. Let me answer your questions.

Do your sculptures ever place wood in a situation where it cannot move seasonally?

No they are all free turned pieces, free to move.

Do you ever combine woods in cross grain situations, for example the end of one slab firmly joined to the side grain of another, where one piece tries to move seasonally and the other does not?

No but they are sometimes loosely connected by free dowels.

Do your sculptures ever combine different species of wood where different rates of seasonal movement will conflict?

Not in any tightly connected way

In other words, do you need finishes that most successfully resist moisture movement above all other finish characteristics?

No I guess not. What I have in mind is the idea that many wooden structures such as natural unpainted, wooden buildings last for decades. I would like my sculptures to do that!! In other words they are entirely compatible with moving back and forth as you describe (that includes natural cyclical minor/minimized splitting, distorting etc -. But what I would like to preserve (and occasionally refurbish with time) is the appearance of the natural grain or its dyed equivalent. So it is the weathering change on the granular or micro scale that I am concerned about.

Thanks

Re: outdoor finishes???

Don Stephan

My apologies if again I am offering what you already know.

Some woods are more resistant to weather and deterioration than others. Walnut and redwood heartwood are rot resistant, but I doubt their sapwoods are. White oak, cypress, and locust are generally considered very weather resistant, no idea if their sapwoods are. I believe south american purpleheart is very weather resistant, but the purple color will turn brown. Of course teak is very weather resistant, but it will naturally turn gray.

So if you can use these woods and others like them a finish may not be necessary.

What I have read of spar varnish, often used on exposed boat woodwork, is that it needs annual light sanding and re-coating, and every 5 or so years stripping and starting over. Again from reading spar varnish is an excellent outdoor finish because it does not cure hard/brittle, so it can move slightly with temperature and other environmental conditions (maybe with seasonal wood movement as well).

UV additives in finishes slow the damaging effect of exposure to ultraviolet light, but do so by themselves being destroyed by the ultraviolet light, so they are effective only for a while.

I'm waiting for others with outdoor finish experience to contribute.

Re: outdoor finishes???

mikeee12345

Boiled Linseed Oil would be a simple oil finish to apply once or twice a year to renew the wood.

Don, No sapwood is considered durable,

Keith Newton

They are all prone to rot / decay quickly compared to the heartwoods which contain many extractives which usually make the wood more resistant.

Re: outdoor finishes???

Peter Rand

Don - no apologies needed - your info is valuable, thanks. Coming to the conclusion that what is realistic is to preserve the integrity of the wood for some decades, and tolerate some "weathering'' of the finish!

mikeee - yes it appears film finishes are not much better in that they may last slightly longer but are much harder to renew

Keith- will try to stay away from heartwoods but I must say some of my pieces retain bark!

Thanks guys

Re: outdoor finishes???

David Walser

Peter -- If you want the wood grain to show through the finish, then you should look to marine grade varnishes for your finish. There are a number of different brands and I've not used enough of them to make a specific recommendation. Marine varnishes are designed for very tough environments -- wet, salty, lots of sun. It won't hold up forever -- the crew of a large yacht will be almost constantly renewing the finish somewhere on the ship -- but it does offer much better protection than the finishes we commonly use on our turnings.

Another option is an epoxy finish. My experience with these has been mixed. I've used them when I needed a water-proof finish. My problem has been that the finishes tend to crack with wood movement over time.

HTH. Maybe others can chime in with specific brands that have worked well for them.

Re: outdoor finishes???

mikeee12345

True, a marine grade resin or epoxy finish will stand up to considerable outdoor weather.

The one drawback is when you need to refinish the item you need to prep or strip the item

to get a good adhesion for the new layer of finish. Many of the polymer finishes will end up

yellowing over time which will be another issue to contend with.

Re: outdoor finishes???

Peter Hyde

Peter,

as we are only a few klicks apart geographically I can give you some insight.

I have front steps made from cedar and spruce as a temporary thing about 25 years ago. To give some protection i brushed on some BLO diluted about 20% with turpentine to aid penetration. Steps are now dark brown/black and still going strong with virtually no checking or rot.

Main garage door is made from pine and MDF and painted. Paint peeled badly around stiles and rails and showed signs of water penetration into MDF panels.

I applied an equal part mix of BLO/spar varnish and turpentine intending it to offer some protection until I could get some fresh paint on it. Well it's still good after 5 years and no further deterioration to the exposed MDF. One day I'll get the paint out!

The only negative is BLO makes wood go dark and there is no shine or sign of surface film.

Re: outdoor finishes???

Peter Rand

Peter - Very good to hear your experience! It seems to be coming down to oil penetration and not film finishes. Dilution for deep penetration to begin with. Of course you had good woods to begin with but I'm also looking at those 85 year old untreated cedar grape posts around here that are still in good mechanical shape still!! Just gray. What if they had been oiled. Thanks for your input.