I would not use pvc board for a deck, but have used as a core it for smaller projects. It is nothing like starboard and accepts resin quite well. As with any smooth surfaced material you have to rough it up to get a good tooth for the resin. In my experience sanding works well enough for this.
There have been tests bonding to PVC that showed solvent wash and flaming the surface with a torch produced a bond that resulted in substrate failure, and OVC primer and solvent prior to bond produced Results nearly as good.
https://www.epoxyworks.com/index.php/bonding-pvc-plastic-with-epoxy/ Note: even the results where the bond to surface failed are not disqualifying because such failures are common with all foams including COOSA, because the resin doesn’t penetrate very deeply into the surface. That’s kinda the reason we choose foams because water doesn’t penetrate for the same reason.
As I said, I wouldn’t use PVC board for a deck, but you have lots of other choices that cost less than COOSA. These include Dinycell, Airex, Nidacore. All work well for decks, cost less than Coosa and are used by boat manufacturers for decks. My choice would be whichever I could pick up within reasonable driving range, because freight costs for sheets are brutal.
By the way Divinycel is PVC, so it is chemically the same material. The foam board has a smaller celled foam structure and a molded smooth surface that has to be broken to expose the core. I’m not sure where the myth about a PVC not bonding comes from, maybe improper surface prep, but it’s untrue.
