I am wanting to use PVC expanded board to make up the backing for sunpad and bench seat. 1/2" marine board is strong enouch, but is 1/2" PVC expanded board going to be strong enough? Do I need to use 3/4" PVC board?
Any thoughts or experiences.

The board will outlast the upholstery... I'd go with marine or treated plywood.

That said, you only need the 1/2" because the whole thing sits in that fiberglass tray.

Not to change topic but tell me more about PVC board. i build outdoor kitchens out of starboard and the $300 a sheet is usually a deal killer for the less than rich. Is the finish good enough to use on exposed surfaces? Cost? Easy to machine? Thanks.