I'm about to tile my bathroom floor but the floor is concrete and it is not flat. It still has the thinset trowel markings on certain areas and they are not in the same direction. How do I even out the floor?
Concrete – How to remove old tile mortar from a concrete floor
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You probably don't need to concern yourself. The thinset plus tile should smooth out the imperfections just fine.
If you really want to remove them, you'll probably need to use a concrete grinder. But that's pretty expensive. As a "lightweight" alternative, a 4" angle grinder with a masonry wheel would knock the top off of the ridges. But it would take a lot of time and make a lot of mess to really remove much material that way.
If you want to ensure a perfectly straight and level corner along the entire length, then I'd suggest you build a two sided "box" to rest on the raised surface. Build it out of 1x1s, making a frame that consists of 3 long straight edges connected at perfect 90 degrees by short supports (cripples) spaced 12" on center.
Lay that box over the concrete edge, and use shims to get it perfectly level. Glue in place.
Then apply plywood to the frame and proceed with tiling as normal. (You don't need to ditra the box in this scenario, just tile to the ply)
Rough sketch showing cross section of wall/floor and stick diagram of proposed box framework:
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Best Answer
Thinset mortar is usually fairly soft. For that small area you can probably knock down the lumps with a belt sander and coarse grit belt. You don't need to entirely remove the old mortar if it's well bonded. Just get a fairly flat top. Vacuum thoroughly before proceeding to remove all dust and debris.
Since you're presumably going to use a 1/4" notch trowel to set your tile, the mortar will accommodate the remaining roughness. It'll be a bit more difficult to apply the mortar than on fresh concrete, but no big deal.