We're making a little server cupboard over our stairs in some dead space. I was proposing to just bottom it out in ply and paint it but mrs ik9000 wants a plaster finish if possible. Is there some lightweight plasterboard for overhead use or should I just skim the plywood (hopefully lighter weight as there is not much to fix heavy PB to)? Not a huge area, circa 1m x 0.65m from memory. Can you just skim plywood or is there something I should do to it first? Or is it just a bad idea? (I've got an offcut of some 12mm marine ply from an old job and was proposing to use that.)