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Old 05-11-2007
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Treated Plywood

What's the difference between so-called "Marine Plywood", and the regular treated plywood you can buy at any home improvement store? Is there a difference?
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Old 05-11-2007
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Yes there is!

I asked this one before my recoring project last year. The conclusion was to definitely use marine plywood, which I did. The main reason is the strength of the marine ply vs treated. Treated ply is flexible vs marine ply. Marine plywood is made up of more layers than treated ply of the same thickness. Each layer is also made of a harder wood. This results in a stronger section of wood. Nothing beats having strong wood! (Sorry, I couldn't resist)
Anyway, stay with marine ply.
FYI, you also won't have to worry about the nasty chemicals that come in the treated ply.
Russ Duff
Catalina 38, Hull #112
Lake Erie
Grosse Ile, Michigan
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Old 05-11-2007
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Just remember to fully encapsulate it, otherwise it may (eventually) rot. Also, if using it for deck core, use the "epoxy donut" method of oversizing the through-holes.
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Old 05-11-2007
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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.

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Old 05-11-2007
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AFAIK "Marine" also has zero voids in it, making it more reliably uniform and strong. Then there's also "Bruneel" ply, which is a marine plywood which is VERY dense.

What you can use--or can get away with using--or should use -- varies with the job.
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