plywood grade:
A - flawless, sanded smooth, cabinet/furniture/varnish grade
B - some small blemishes, knots or holes filled with matching putty
C - all imperfections have been cut out and "boat" type fill plugs installed. Paint Grade.
D - knots and splits are open, mostly the back side only. (gnarley).
X - exterior grade adhesives used - somewhat weather resistant if sealed/painted correctly.
so, CDX would be smooth one side, gnarley on the back side.
designed for "in the weather" applications IF sealed/primed/painted correctly.
CDX-SYP is the most common for outdoor applications. SYP stands for Southern Yellow Pine.
CCX would be more rigid as it has less imperfections - but difficult to find.
MDO (Medium Density Overlay) plywood is made for exterior signs. Fabricated with
exterior adhesives, has a layer of brown paper adhered to one or both sides,
can be found primed or unprimed in Sign Supply places and sometimes at the Big Box Stores.
avoid MDF (Medium Density Fiberboard) unless you are building kitchen cabinets.
avoid pressure-treated plywood and support lumber as it has a tendency
to corrode and destroy metal—up to five times faster than untreated wood.
avoid OSB (Oriented Strand Board) as its sole purpose in life is for roof decking. (not boat decking).
as mentioned, there is a 1/2" exterior grade plywood that works well with
boat floors IF properly supported, sealed, primed and painted prior to carpet.
I don't know about other stores, but, my Lowe's here has signs on its plywood that
simply says "for out of the weather" applications and "in the weather" applications. (such as a doghouse).
Please don't take the word of the "Lumber Yard Professional" to guide you in the right
direction..... all he is doing is regurgitating information that he himself has read on the signs.
and, welcome aboard !!
Personal Note concerning plywood:
Plywood is fabricated in a plant, all pieces are cut and glued together then stacked
in a bundle and steel straps applied to keep it all tight and uniform for storage and
shipping. When it gets to the Big Box Store, it is still stored in the bundle under
tremendous pressure - - - - with this being understood, when you select plywood and
take it home, this is its very first time to "breathe and stretch" since its beginning.
Unless you fabricate it to your project right away, it will start to bend, twist, cup, warp,
and take on shapes other than for what it was intended to do......
So if you are not going to use it right away, I strongly suggest laying it flat on a floor
with some weight on it to minimize this physical movement.
If you just lean it up against the wall and "get to it later" - - - It may (or may not)
be less than flat and straight for your project. And you may (or may not) achieve less
than favorable results in your projects.
This, of course, depends on your location, your temperature, your humidity, the humidity
content of the wood, yada yada yada.
jus my Dos Centavos
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