Shortly after moving into my new house I realized that whoever worked on my fireplace originally had absolutely no clue what they were doing. The brick portion of the fireplace had 4 holes cut into it, I understand it is for better air movement into and out of the firebox (that isn't the problem). The brick bows out in the slightly in the front above the firebox opening. Also the brick bows, slightly more, in the legs where the firebox opening is on both sides. The legs are probably 1/2 in, maybe a little more off from being square. I plan on tiling directly over the brick with marble. I know that I'll have to first patch the holes...but I'm not sure what to do after that. From what I've read, you should apply a coat of thinset or mortar over the brick to create a smooth, flat surface to tile over. Can someone please explain to me what the best way to accomplish this task is with the given difference in the legs and top of the firebox?