Join Date: Feb 2000
Location: Annapolis, Md
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There are several big differences between marine grade plywood and treated plywood. To begin with treated plywood is typically APA construction grade plywood. Construction grades generally have fewer internal plys making the plywood more prone to warpage and face splits. Construction grade plywood allows considerably larger percentage of voids and internal defects than marine plywood and typically has C-D grade faces. These voids greatly increase the chance of delamination. Marine fir plywood uses the same species as fir treated plywood, but most treated plywood is a fast growth pine. Treated lumber does not accept paint, nor polyester or epoxy saturation as readily as marine grade material. This makes treated material totally unsuited for core repair or laminated structures. Because of the C-D face plys treated plywood never quite looks finished and so should only be used in hidden locations.
There is minimal difference in strength or stiffness. Frankly I would almost never use plywood for coring. Because of the fiber orientation of plywood, rot spreads in all directions. The fibers in organic coring are generally oriented perpendicular to the deck greatly reducing the speed at which rot spreads.