posted 9 months ago
Inasmuch as it's going to be subject to the elements, I'd opt for the easiest to maintain protection. Accordingly, I'd avoid surface coats. In part because they expand and contract at a different rate than does the wood they are applied to. This causes them to fail quickly. And the cracked and split surface coat, after letting water in, can slow it from evaporating off again.
To reduce this problem in nautical applications, they use what is called a long oil finish. All that means is, they added more hardening oil than they would if the finish was being applied to the wood floor of your house. This results in a softer, more flexible finish, so it shifts more with the wood. Unfortunately, it is far more expensive than so called short oil formulas (the floor formulas that are harder). And it still isn't bullet proof.
Merely that that can of Flecto Varithane says it's for exterior applications doesn't mean it's going to be the magic bullet.
IF I did go with an exterior poly, it would be oil based, for the reasons noted above, and I'd thin it at least 20 thinner. Then I'd plan on spending an hour working the finish around, to keep the surface wet as long as possible. Since the poly finishes harden by reaction with the oxygen in the air, keeping the poly wet gives you you more time to get thinned finish into the wood, rather than resting on top and creating that future cracking problem mentioned. While doing this, you would focus on spots that sucked the finish up, like knots and end grain.
I've done this with burl slabs I was making into tables, back in the day. I large slab would suck up a few gallons of finish, but withstood forty years in front of a fire place in Bremerton, Washington. The penetration was so complete, when I dropped a paint brush I was using, I looked up and could see where some of the finish had soaked through the five inch slab.
Another thing to remember is, wood, generally, has six sides. If three to five of them are left exposed, they will act like water magnets, as the wood tries to stabilize to its surroundings (the humidity). For this reason, if furniture were sealed on all surfaces, you would get slower absorption and loss of moisture, so moving that piece of furniture between West Virginia and California would be less likely to cause damage, as the furniture expanded or contracted with the humidity changes.
One protective coat you could use is, highly thinned oil, like mineral oil and the seats and tops, then fresh automotive oil on all the framework. Thinning it goes a long ways to cranking up penetration. Let it sit and much of the oil will continue wicking deeper and deeper.
The oil applications are cumulative. The first one may not look like you've done anything a few months in. Aggressive applications and, three or more in, and you can see something was done even months later.
If the oil treatment were done on thing shakes and shingles, cracks and splits would seem to disappear, as the wood swelled back to its original state. And the shingles would remain more resilient and tolerant of being walked on [without breaking].
If, when you tire of applying finish and waiting, it's still oily, grab some dish soap and wash it off. Why worry about the water, it's going to get hit by worse. Right?
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