Is marine grade plywood specially treated to resist salt water environments or severe exterior exposure? NO. Okay then what is marine grade plywood and why would it be used?
Marine grade plywood is a special grade of softwood veneer core plywood as defined by APA - The Engineered Wood Association and Voluntary Standard PS 1 "Structural Plywood." the standard is available as a free download from APA after registering.
Marine grade plywood must meet all the requirements for Exterior plywood plus the following:
- Only Douglas-fir and Western Larch plies without knotholes are permitted in the plywood construction.
- The panels are available with sanded A-A, A-B, B-B faces or HDO and MDO faces, only.
- The standard limits repairs in Grade A faces to a maximum of 9 in a 4 x 8 foot panel.
- Inner plies to be minimum Grade B.
- Crossband gaps and edge splits and end splits and gaps are limited by dimension and number.
So there you have it. No special preservative, no special adhesive. Merely limits on the veneers. If decay is a concern, even marine grade panels should be preservative treated.
Other EXTERIOR plywood may contain plies of other species, Grade C plies that do contain knotholes, and larger gaps in the plywood core.