I want to put subway tiles on the walls around the tub in the bathroom of my 1924 Bungalow-style house. The walls are very thick and tough grey, cement-like plaster with a white finish coat. Can I tile directly over this, or do I need to remove all of the plaster (a tough job!) and install backer board? This wall is VERY hard to cut through! Also, if I do have to remove the plaster, do I have to remove the whole wall even though I am not tiling all of it, so that the wall will be flush?