Agree with you Johnno,,,,, as I'm a carpenter is my field, marine ply does actually have better weather/water resistant glues that bind the laminations than form ply.
But form ply has the surface impregnation that's waxy like and repels water (black film).
There are two gades of form ply with one being a better quality,,,, the better one usually has grey color weather protective edges while the cheaper is green color edges.
When you cut the ply, it's best to seal the cut edges against the weather to bring it to same standard as the rest of the materials surfaces.
If marine ply had the same protective or good as coating as form ply, then marine ply would better it.
Reason for marine ply being so much more expensive than other plys, is not only its glues and usually select twin faces, but it has a solid cross hatch stitched construction with no ends or little gaps (spaces air pockets inside) making it a proper construction grade with no particular compromised areas.
If any type of ply used outside, go ply then foam then a black plastic sheet a inch larger all the way round,,, this would make a little skirt so to speak to direct water away from edges and possible capillary action of water (gravity defying water travel).
Edited by Buccal, 28 July 2015 - 02:18 AM.
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