Thanks SidecarFlip, your comments are generalized but welcome.
I intended to coat all plywood surfaces, especially all exposed or cut edges. This particular product says it is intended for use prior to staining but also says it is for surfaces not intended for painting which makes me question adherence characteristics of the treated surface. The plywood underside will be in contact with the polystyrene, likely with some construction adhesive, and I was thinking of vinyl type flooring on top glued down with some leftover proper flooring glue from my kitchen reno.
I agree with the OSB comments from the research I've done which is why I intend to opt for Douglas Fir plywood as my choice.
