Pressure-treated plywood, often called "Wolmanized" or P.T. plywood, is NOT " Marine grade" plywood, and those designations do not make the two products arbitrarily interchangeable.
Pressure treated plywood is common plywood that has been subjected to pressure treatment with chemicals to prevent the wood from decaying, or rotting. To some degree, it also discourages insect damage because of the chemicals involved . Pressure treated plywood, however, is not suitable for marine use. The treatment of plywood with copper and arsenic compounds under pressure simply does not make the plywood waterproof, and worse, continuous exposure to water will leach the preservative chemicals from the pressure-treated wood.
Again, pressure treated plywood is ordinary, interior-grade plywood that has been chemically-treated, and it is often made with softer woods to enable the penetration of the wood treating chemicals, with no special care effected to eliminate all gaps or voids.
Exterior grade plywood is made with water-resistant glue, but the exterior shell is the only layer that is made void-free. There may be gaps, voids and the resulting points of weakness in the interior layers. When you cut a sheet of exterior grade plywood, you may expose a gap on the cut surface.
Marine grade plywood, on the other hand, is a different creature. Marine grade plywood is assembled gap and void-free in all layers, and laminated together with special, water-proof glue that holds the various layers together. When immersed, water has absolutely no effect on the glue or the strength of the lamination of marine grade plywood. Marine grade plywood will not commonly delaminate, bubble, buckle, or warp. Upon cutting marine grade plywood, no voids will be discovered on the cut edges. It is also usually constructed of harder woods such as Douglas Fir, or Western Larch.
Marine grade is a superior grade of plywood, and a substantially better product.
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