If your situation allows it, how about 5/4 boards like
are typically used for decks? No coating needed.
They're waterproof, strong, have rounded edges, and
are small enough to be easy to manage while building.
With some 2x under them, I've used them for a lawn
tractor ramp and a 12' high play fort and they're going
on 20 years.

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Awl --
I'm looking to make a 4 ft x 6 ft base for some apparatus, out of 1/2"
plywood, for outdoor use.
It works well indoors, but I'm wondering if I can make it *truly* waterproof
for outdoor use, with enough coats of poly, epoxy, or some clear plastic
coating-type finishes I've seen.
Not that familiar with wood/coatings, beyond having done my floors with a
water-based poly+catalyst, with mixed results.
Also, I know there's HD 1/2 plywood, which you can break across your knee,
and there is real 1/2 ply, from a lumber yard. Are there even
harder/stiffer grades? I'm not necessarily looking for furniture-grade
plywood, but maybe that is indeed the stiffest. Cost, bang for the buck is
a factor.
Maybe other "engineered products"?
Appreciate all input.
--
EA