If you are going to tile or otherwise cover a concrete floor in your home and the surface is generally in good shape but contains some dips, depressions, holes or cracks, you need to get it filled and levelled before you proceed. The best way to do this is with levelling compound. This is a thin, watery form of cement that levels itself out (like water would) when it is applied over a concrete surface. It dries quickly, so you might want to practice working with it on a hidden surface before you start.
Remove any debris or loose concrete. Thoroughly clean and dry the floor. Lay your 2-by-4 inch board over the surface and scoot it around the area to get an idea of which sections are dipping below the main level of the floor.
Roll sealing primer over the whole floor with a floor roller. Make sure to get it down into the low spots and crevices. Let the primer cure for 48 hours.
Mix your leveller with water in a bucket, according to the instructions, using a mixing bar attached to a drill. The compound will be thin and watery. Do not mix any more compound than you can spread and level in about 10 minutes.
Pour the compound over the low spots or cracks and holes in the floor, starting at the corner furthest from the room and working in sections of a few square feet. At each section, spread out and level the leveller with your cement trowel, getting it down into the depressions while flattening out the top so it is even with the surface. Work quickly.
Let the compound set for several hours. The floor is now ready for tiling or other finished flooring.
Wear a dust mask when mixing the leveller.