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Old 04-28-2009
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How can I tell my plywood is Marine Grade?

Please forgive this "newbie" question but i just special ordered marine grade plywood from my local lumber yard. I brought it home and it has no stickers or any engineering information on it or with the receipt. The pieces do have some purple stripes on the edges. How can I be sure that the plywood i ordered is indeed marine grade? This stuff is very expensive and i just want to feel certain i got what i paid for.

Thanks!
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Old 04-28-2009
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There should be no voids (air pockets), any knots should be patched, and it uses a different type of glue that's supposed to be water proof. Both sides should have a smooth finish. Google marine grade or Grade 'A' plywood and you can find more info.

PS - ya, the stuff ain't cheap.
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Easy way to tell, cut a small piece off and drop it in a pot of boiling water, and turn off the heat. Ordinary plywood will warp and delaminate in boiling water, marine plywood will generally withstand it for a lot longer.
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Old 04-28-2009
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thanks guys! i am aware of the differences i just cant tell by looking at it. just looks like regular plywood to me. i did cut a few pieces of my first boat and i did not see any voids.

sailingdog...thanks for your reply..just so i understand....bring the water to a boil, drop the piece in and remove from heat? i dont actually have to boil the piece of wood for say...2-5 minutes or anything?
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Try it with a regular piece of plywood and then a piece of the Marine stuff... you'll (hopefully) see a fairly significant difference.

One other point... good marine plywood generally has much thinner plies than does normal plywood. If you compare the two, the marine plywood will generally have 40% or so more plies that are 40% or so thinner...

Look at this cross section of Regular Plywood with finished faces, see how thick the central plies are. Marine plywood of the same thickness would probably have seve or nine layers, versus the seen five here.

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Your some what limited in what you can prove or disprove now. If you bought American graded plywood then it could be C grade innner ply's for all you know. We don't grade then innner layers. The British system rates all layers but it uses a numeric system. You most likly bought either A/C grade or A/B grade. Add in that the football patches that are on the B or C grades aren't usually glued on the edges very well. And marine grade used to only cover fir plywood but southern yellow pine is also sould as marine plywood by a lot of lumber yards these days. Do however remember that there is a big difference between A grade and B grade veneers and that alone makes for a big difference in price.
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Old 06-16-2009
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The good stuff has a stamping of the Lloyds Register Approved,
tested to :BS 6566 exterior , water and boil proof (WBP), exterior veneer thicknesses are typ around a .040" to .060" thick, no voids....
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