I have been asked to put a 8 inch (2 rows of 4x16) subway ceramic tile stripe on a drywalled wall. It's aesthetics not protection of a wet area. I have run into an issue where the wall has a bad concave of about 18" long and 1/2" deep in 1 area. Whats the best way to get this filled in to get the tiles running flat. I was thinking of feathering drywall mud in the area and priming it but not sure the tile would end up sticking to it. Since it would be so thick.Then I thought mortar but would it slump and not stay in the area till dry.