I have an older home with a bathroom that has a tub and I want to add a shower. The tub is surrouned by nice tile set in concrete, bu the tile only extends about 3 feet up the wall, then there are plaster walls up to the ceiling. The top row of tile is curved to blend into the wall so it's not like it would be easy to add to, plus the tiles are an odd size. Is there some product I can put on the plaster walls to waterproof them for the shower? Thanks.