Does anyone know exactly what the difference is between marine plywood and interior plywood? Both use exterior grade glues on the substrates. My guess is the filler materials are different. If I build an outdoor bar and seal it will varnish, etc., won't they both hold up the same, since the finish will prevent water from entering the substrates?
From contributor H:
Exterior grade is not the same as waterproof. I believe the glue used in marine grade ply is different. Another difference is the amount of void allowed in the cores of the plywood. Voids are where moisture can get trapped and are weak spots in the sheet. Marine plywood is made entirely of Douglas fir or western larch. Glue is completely waterproof. Grading of all plys is far better.
With marine grade, the lowest grade veneer is "B", which means that there will be no serious voids on the surface and interior plies. It is the voids that cause poor adhesion! So, marine grade will have good adhesion throughout the lamination. Also, marine grade can be cut without getting a void in one laminate on a freshly cut edge. The edges will be totally solid. The two species used for marine grade are Douglas fir and western larch, although I have seen keruing and other species used in a product called marine grade. You can get the same performance from panels such as exterior AC in most cases.
Marine grade has no natural decay resistance. It has no chemicals added to enhance decay resistance, unless it has been subsequently pressure treated (= $$$). Marine grade has no special waterproofing in or on the wood.
If you do not get the wood wet very often and if it is sealed, as you state, there is no difference in performance indeed by using a different, less expensive grade and species. Note that most interior grade panels do not use exterior adhesives. But if the plywood seldom gets wet, the interior adhesive will be just fine.