Hello, my wife and I are preparing to install a tile shower surround in one of our bathrooms. Last night we removed the old plastic shower surround and found that the walls behind the old surround were plaster walls just like the rest of my bathroom. My question is after I removed all the plaster from the walls can I install the new tile surround directly onto the surface the plaster had been on or do I need to remove everything down to the studs and put up new cement backerboard? Thanks!