The walls surrounding my tub are finished with 4×4 ceramic tile.  The tile is in good shape but the tub isn’t level, so the sealant joint at the bottom edge varies from 1/8″ to 1/2″, which looks lousy and is prone to adhesion failure and consistent bouts of mildew.  I was thinking a solution might be to add a 2″ hi edge tile along the base of the wall and trim each piece to length so I end up with a consitent 1/8″ sealant joint around the perimeter (the top edge of these tiles would have a bullnose to ease it back into the existing tile visually).  Is it feasible to put tile over tile on a wall in a wet location?  Do you need to use a special adhesive, and is cleaning or roughing the surface of the existing tile required to ensure adhesion?  Thanks.