"Wood is good" unless it fails to serve the purpose for which it is intended To understand the differences between grades of plywood, and the meaning of the various designations, is to choose wisely and appropriately. The saying " Be fooled at your peril" applies to many things, but when considering the construction and end uses of various types of plywood, if you are fooled into using the wrong grade, the results can be unnecessarily expensive, or even disastrous. .
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.
G1S plywood, (good-one-side) is plywood with one side graded "Select" to show no defects or gaps and is an aesthetic consideration.
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.
Do choose carefully when selecting plywood for marine use. Although it is more expensive, marine-grade plywood, when finished appropriately, will outlast pressure-treated plywood by far. The ordinary glues used in plywood , pressure-treated or not, will eventually fail for structural reasons.
When the transom on your boat fails in the middle of the lake, the wisdom of having saved fifty dollars by buying cheaper pressure-treated plywood instead of marine grade will come to question rather quickly. In this application, and other critical structural applications, let us suggest that "the RIGHT wood IS good", and marine grade is best.
Now you know the difference between marine grade and pressure treated plywood.