So, I'm in the midst of a pseudo-prop construction and I have a question for all you wood-oriented proppers.

I'm a tad nervous about risking chemical reactions that would ruin my prop. What could I use to seal some plywood and other lumber against weather, while still being able to safely paint and bond-o the outer surfaces?

I don't think you should worry about treating the wood you will use for your pack, unless you feel a sudden urge to go dancing in the rain. Plenty of primer and paint should be enough for your pack, and if you want then spray on a dull, weatherproof, topcoat to protect it. Otherwise your pack should be fine.

Who said anything about a pack? =o)

I'm in the midst of building an ECTO-Sorta (The M.A.U., actually.) and this stuff will be exposed to the weather.

The Proton Pack comes later.