I prepared my tub walls for Swanstone but than someone decided we wanted to do tile instead. Seams are taped and spackeled and everything is flat and ready. Can i tile over green board? I remember taking an old bath apart and finding the tile had been glued up on greenboard with mastic instead of thinset and grouted... it had worked just fine. As an alternative i was thinking maybe just he 1/4" hardibacker over the greenboard? What should I do?