My advice:
One thin wipe of water-based polyurethane, then immediately dry off the excess with a clean dry, lint free rag.
Do this once per side - wet one side then dry it, then move onto the other side.
Then once it's fully dry, give each side a very light rub with 220-240 grit sand paper (it will feel touch dry in 5 minutes, but give it a rest overnight to dry properly). Remember you are not trying to remove *any* of the poly here, you are making the surface *just* rough enough to glue the rubbers on afterwards. Better to go too light than too heavy with the sanding (-- if you find the rubbers aren't sticking properly you can always sand the surface a little bit more.)
Follow the above method, be sure to use water-based poly, and you should have no problems.
(Plus, if you're also willing to send me a few close up pictures of the ply sandwich's glue lines when you get it, I would be very grateful. )
Good luck with it. 🙂
