I’m building a telescope. The mount is largely made of plywood, with the base on hockey pucks used as feet, so it’s about 1 inch above the ground. Obviously in this position, the plywood bottom is highly likely to encounter moisture. Plywood does not react well when the edges get wet; it swells and cracks. Of course I will paint the plywood but I’m wondering what I can do to increase its water resistance before I paint it. Part of the problem is a plywood edge will certainly have small voids at various spots where an individual ply is defective. I’m thinking of scraping a layer of Bondo around the edge before painting, which will fill the voids but I’m not sure whether it will be of any use on the non-void part of the edge. Any other ideas?
Spray-on truck bed liner? It’s waterproof and near-indestructible.
You can’t seal wood before painting. Paint needs to bond with the wood before sealing.
I’d cover the painted wood in Minwax Helmsman Satin Oil-based Spar Urethane. Check the can first and confirm its ok on painted surfaces.
Put on at least three coats.
Reapply every couple years.
Spar Urethane is one of the toughest exterior finishes that I’ve used.
You could try West System Epoxy.
Along the lines of spray on truck liner, you might try some (spray) Plasti-Dip. I just did the rims on my car, it went on very easily and while I don’t know how it’ll hold up over years of driving*, I’ve driven the car in the rain multiple times (including 6 hours later) and so far it’s fine.
Have you considered getting a solid piece of wood? You should be able to find one the same size and thickness as the plywood and throw a coat or two of exterior primer and paint on it.
If the only thing you’re worried about is some moisture from the grass(?), you could probably just leave the plywood unpainted. I’d bet it’ll be fine for years seeing only a little bit of dampness here and there, it’s not that it’s going to be outside permanently. From that perspective, it may not matter what you paint it with.
*Most people say it should last about 3 years on a car.
See if you can find any marine grade plywood. It’s made to stand up to an environment with a lot of water. The advantage would be that even if water gets through whatever sealer you use, the plywood itself would resist the water. Typical plywood will fail once moisture gets past your barrier.
The base is a 25 inch diameter circle. There’s no way to find a solid piece of wood that wide. I could glue up a panel, but a solid panel is much more likely than plywood to warp. That’s kind of the point of using plywood, it’s very dimensionally stable compared to solid wood.
You’re not using Baltic birch plywood? 
:eek:
CMC fnord!
Yes as a matter of fact I’m using Baltic birch plywood. Why do you ask?
I’d hate to think that someone was going to the trouble of hand building a telescope and using the wrong materials!
Let’s talk about the “plywood.”
There are essentially three kinds:
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OSB. Do not use USB. It sucks when it comes to moisture.
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Traditional plywood. Much better than OSB, but moisture will make it warp.
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Advanced sheathing material. The best is AdvanTech[sup]®[/sup]. It’s not cheap, but it is the most moisture-resistant sheathing material you can buy. I think The Home Depot carries it. And if you want the ultimate in moisture protection, seal the four edges with epoxy, and then paint both sides with epoxy paint.