Originally, Alumacraft used white oak on the inside and plywood on the outside. Both boards had a cork gasket between the board and the aluminum. If you do it the original way, you won’t need sealer…as the cork acts as a gasket and also dampens vibration. White oak is a very hard wood, so it’ won’t crush. Aluminum on the inside/outside increases the risk of the outboard walking off the transom in my opinion. All three of my Alumacrafts have had the transoms reinstalled the same as the factory. I used spar varnish to seal the boards. One of my boats transom lasted over 50 years…so they had a good design. I highly recommend using the cork like they did at the factory. It’s a very good design.