Installing New Tile
Not sure how wrong this may be, so let's hope that Mike is still around to tell you it's nuts, but I'm old school (too old school sometimes), so when we retiled our mechanical room I gave my helper (a.k.a. wife) a fire brick, relatively flat with a mildly coarse surface, to knock off the high spots, and it worked great. Once the ridges were knocked down, we cleaned it good, checked it with a straghtedge, as Mike mentioned, and started laying tile. I don't know if the scuffing she did with the brick was absolutely necessary, but I felt better knowing that we had the subsurface relatively flat.