We're putting up a glass tile backsplash and we ran into a bit if a problem. The kitchen is in the lower level of our bi-level house and where we want to put the backslash up is where the concrete from the below ground portion of the house meets the drywall of the above ground portion. The previous owners masked it with a chair rail, but basically the concrete protrudes about 1/8 of an inch in some parts and 1/4 of an inch in others. At first I thought we would just add backer board to the drywall but because the difference varies so much it may not actually fix the issue. Do you think mastic (or some other thin set) would be able to fix this? Any other suggestions on how to even out the two wall surfaces?