I agree with Don. ( I have to, he laughs at my jokes.) Epoxy is used to encapsulate timber. That includes the inside of any holes drilled in or through the plywood. You certainly don't add fillers, colloidal or otherwise when coating, as you want minimum viscosity for maximum penetration into the substrate. Coating on one side will prevent moisture drying through that surface. It's expensive and tricky to totally encapsulate, so I would go for some form of preservative soaked into the plywood. There are previous threads on this. Try http://www.airforums.com/forum...ad.php?t=14506
Depending on which preservative is used, you may need a gasket material to prevent contact with the aluminium skin.
Mixing of WEST epoxy is best done in small amounts, perhaps a cupfull, and mixing is best on a flat dish like a tray, with plenty of surface area exposed to the cooling air. If mixed in a cup, the exothermic reaction will quickly boil the mixture, and the very hot container will go solid as the mixture sets. Been there, done that!