We are putting congoleum tiles down on concrete flooring through a hallway and bathroom. What is the necessary prep work to be done to make the floor last longer and look good. After plucking off the old congoleum squares, there is a black painted surface over the concrete. Do we need to apply any floor leveler to this?There is also a crack in the concrete at one area with about 1/4 inch difference in height between the two levels. How do I level this off?