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Sign board is a high quality laminated plywood, fir or birch.Google laminated plywood and you should get some hits. Sign board is a mid-grade of the same wood used for concrete forms.  Xtraply birch PSF (Phenolic surface film) high density overlay for concrete forms, Medium density overlay is "sign board" used for signage and intended to be painted.
I get 1/2 inch HDO (concrete form board) and use it as pallet decking and lids . This wood will not warp, 5/8 or 3/4 lids tend to not blow off  as bad as 1/2 . If you paint or seal the edges it will last much longer, but even without paint it will last as good as PT plywood.



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