The basic difference between marine plywood and exterior plywood is that marine plywood does not contain voids. Edge voids are easily filled with wood putty. Taping the seams and edges with fiberglass will effectively seal exposed edge voids. Both marine and exterior plywood contain essentially the same glues. Because boats of this nature are not left in the water for extended periods of time and the difficulty in obtaining marine plywood, we feel that exterior plywood offers distinct advantages.
Exterior plywood is considerably more economical and is a standard stock item at most lumber yards.
We have had good results with both fir and pine. Plywood is easily joined using a scarf joint, overlapping the separate pieces. There are numerous waterproof and water resistant glues on the market. For the most part, glues, nails and screws hold the structure together prior to tapeing the seams with fiberglass. Once the seams are taped, the fiberglass will provide a strong waterproof joint.