the remaining shower pan after demolotion is copper and has 6 small punctures from tile and mortar removal. Can an epoxy membrane be put over the top of this pan to seal or a rubber membrane or should it be completely ripped out? When installing the backerboard on the walls, should the backerboard go all the way to the floor, then the mortar bed installed for the floor or should the mortar bed for the floor be installed, then the backerboard for the walls down to that level?