Lay some dimensional lumber out in the weather
lay some 1/2" untreated/unpainted CDX plywood beside it and see what happens.
smaller scrap pieces will not warp too bad because of the light stress within the plies.
half to full sheets will be worthless in a week (or less).
CDX is an exterior grade plywood - - - pine veneers with waterproof glue.
BUT-it will still warp and twist if not installed on a rigid frame, such as a deck.
T1-11 house siding is a prime example of CDX plywood.
look at any building with T1-11 siding and you will see vertical stress cracks all over it.
and if it was not properly primed and painted, it may experience "dry rot"
even with the exterior glues in the laminations.
yes, on the other end of the spectrum is that furniture grade plywood used
for interior projects, cabinets, furniture, etc is very stable.
if screwed to a rigid frame, CDX will remain "mostly" flat, but still moves within.
during my sign making career, I have used hundreds of 4x8 sheets of
MDO (medium density overlay) plywood for roadside signs.
some have been in the elements for over 15 years and still as flat as when I put them up.
unpreserved wood is like a sponge - it will absorb moisture and react accordingly.
The preventative measure is to protect it from within with primer, paint or varnish.
anything reduced will soak deeper into the wood fibers thus providing a moisture barrier
with subsequent coatings applied to both sides and all edges.
the reducer will allow the mixture to soak deep into the wood, when the reducer evaporates,
the varnish (or primer) is left behind . . . . then, the wood will be a little more stable.
and like said above, avoid large panels when possible.
-or- use a higher grade of plywood; such as marine grade or MDO signboard material.
many of the Big Box Stores carry the MDO signboards for about $48.00 a 4x8 sheet of 1/2".
MDO is already sealed both sides at the factory - and is strongly recommended on tin boats
for transoms, decks and hatch covers.
does that make sense ?
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