They have their own jointing system for these boards that contains silicon or something to make it moisture resistant. It's a complete t**t to sand as it goes hard like flint. If you were to skim the boards, then the surface would no longer be moisture resistant, so you may as well have used std plasterboard. If you don't stick to the spec and something goes wrong it could cost £££££ to rectify. Actually if you check the Siniat book, I think it says don't plaster them.