Water Penetrating Plywood Deck

I am finishing a remodel and the deck has sat unfinsihed for 1 year +. It is 3/4" plywood CDX covered with a rubber coating and aggragate. I am going to put a urethane finish on it but notice that the underside of the deck must have seen some moisture as there are some water marks. Is this a serious problem or once it is sealed is there something I can do to stop any further deterioration.
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Where is your location . Ive never heard of a deck like that , i think , you will have moisture problems
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Sorry if I seem like I'm being disagreeable, but who builds a deck out of sheet plywood? I would image that if it was a good idea (especially using CDX, which I seem to recall is secret industry code for "Cheapeast Available"), everyone would be doing it. But that's just me.
Water marks and stuff like mold underneath the deck is not good; but methink it might just be nature's way of quietly nagging you that you need to not wait more than a year to actually finish a project you've started.
To stop further deterioration, I would imagine you ought to just get around to finishing building the thing. And I'm sorta mystified over why you're going to urethane over rubber coating and aggregate. But like I said, that's just me.
AJS

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