: Is exterior grade ply good enough for a S+G or do I need to buy marine grade?
: I cant find any marine suppliers around here likely....kirk

I agree that unless you're building a boat you don't care if it lasts or not, you really need to find BS-1088 rated plywood. The cost is surprisingly reasonable when you compare it to plain vc cabinet-grade plywood. 3/4" cherry is running round $115/sheet where I live, and 1/8 equiv. BS-1088 plywood is running $69 for Sapele and $59 for Okoume.

If you're having trouble finding it where you live try looking on the supplier lists at Glen-L: http://www.glen-l.com/ (this is the general site, below is the supplier list specifically)

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