Thinset mortar is used in tile installation and often to cover the joints between backer board and cement board. After backer board is installed, thinset mortar is added between the joints. Even if an even layer is applied, it must be sanded down to ensure a thin and smooth finish. This will ensure the tile or other type of flooring that is installed on top of it will be even and plumb.
Verify that the thinset mortar is covering the seams between the joints. Add extra mortar, if necessary, with a trowel until all the seams are covered. Spread the mortar on thickly. It does not have to be perfectly even.
Sand down the thinset mortar with medium-grade sandpaper. Start in one corner of the room and work from side to side across to the opposite corner. Sand over the entire area in a smooth, circular motion. Continue sanding until the mortar is even throughout.
Clean up all the dust from the mortar and the grit from the sandpaper with a clean rag. Dampen the rag slightly so the excess mortar will adhere to it, and wipe down the entire surface.
Blow off all the remaining grit from the sandpaper with a air blower. Plug in the blower and spray all of the dust particles away from the mortar. Inspect the area to make sure it is clean and even throughout.