Thinset is a type of mortar used when installing cement board and backerboard. It is an adhesive made of cement, fine sand and a water retaining agent. Thinset mortar is often used in tile installation -- applied between the joints of cement board to help cover any gaps and seal the board. After the mortar is applied, it must be sanded down for a smooth surface. This can be completed in just a few minutes by any homeowner.
Apply self-adhesive backerboard tape over all of the seams between the pieces of backerboard. Cut the tape off at the ends with scissors and move around the room until all of the seams are covered with tape.
Open a bucket of thinset mortar and mix the thinset mortar with a trowel. Spread it out over the surfaces between the joints. Cover the seams with the thinset. Add more thinset to the trowel and spread it out evenly over all of the seams until everything is covered. Wait one hour for the thinset to dry.
Sand down all of the seams with medium grade sandpaper. Move the sandpaper in a circular motion over the thinset. Go from side to side and smooth out the surface of the mortar.
Clean up all dust and debris left over from the sandpaper with a clean damp rag. Wipe down the surface until all of the sandpaper grit is removed.
Inspect the thinset and verify it is smooth and even throughout. Using the medium grade sandpaper, sand down any rough spots or edges that still remain.