If you are building one of our Didi range of plywood boats, please also see the FAQ - Radius Chine Plywood Boats page for additional questions and answers particular to building with that method. Marine grade plywood is built to much higher specifications than exterior grade. The veneers are more carefully selected for marine grade to minimise interior voids and veneer repairs during manufacture. Both types are glued with resorcinol adhesive, which gives them the characteristic dark glue line.
More care is taken in the assembly and gluing of marine plywood boards.
The British Standard Specification for marine plywood is BSS 1088. It is normal to see some voids in the inner veneers if you inspect the edges of an exterior grade board. There should be no edge voids visible in a marine grade board. Marine plywood cannot be made from anything other than resorcinol adhesive because it has the best adhesion properties to wet wood. If your plywood does not have dark resorcinol glue lines then it is not marine grade.