Marine Plywood Versus Interior Plywood
From Professor Gene Wengert, Sawing and Drying forum technical advisor
There are two adhesives, interior (which does not stand up to wetting) and exterior (which is not affected by long term wetting and would be called waterproof). Marine grade uses exterior adhesive, which is the same adhesive as used in the cheap CD-X plywood or any other exterior grade or "interior grade with exterior glue" plywood.
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.
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