How do you waterproof plywood and how much?

You do know they have "Wolmenized or pressure treated plywood", if these are not to your liking, you can use a polyurethane but the edges will be your problem areas, the edges have to be completely sealed, (to do this is time consuming) I wouldn't use a deck sealer, this could delaminate the plywood. If the project is indoors, but in a damp or humid area, you can apply a "Vinyl sealer", 2 coats, and then apply a "Pre-cat Lacquer, 2 to 3 coats. Both these products are available for about $28.00 a gallon, AT: * http://www.hoodfinishing.com/