I wasn't thinking i would do a underbelly with plywood. I've had a rethink about the whole thing using aluminum frame and high density poly board as covering. More about that later.
But if I was to go through with plywood I would coat it on all sides each peice before assembly with a latex deck stain. So it soaks in protection, then cover the assembled thing on all sides with truck bed liner. Make sure to glue all the seams when assembling too, so the seams don't crack.
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