I want to tile the ceiling of the shower in my bathroom I am renovating. The ceiling is painted and i need to know if i can tile directly over it. The paint has been there for over a decade, absolutely no bubbling or chipping of paint has ever occurred, and it feels very solid. The house was built in the late 1950's in Philadelphia, and at this time all of the bathroom walls including the ceiling was built with wet-bed mortar over a wire lathe. So, the ceiling is basically paint over mortar, very solid.
Should i rough up the surface first?
Or Should i sand it down lightly?
Should it be primed?
Or a layer of Redgard?
I have been using TEC 3N1 Performance Mortar for all tile work so far. Is this Thin-set ok for this installation?