Prime a concrete subfloor in the areas where you will be using self leveling compound. Self leveling compound is used on an old flooring surface or on a subfloor, before installing new flooring. Any dip in the floor 1/4 inch or greater should be leveled before installing new flooring. Self leveling compound can be used on any flooring surface or subloor. When wet, it is a great deal like cement or concrete. Spread it evenly with a trowel, and check often with a level to be sure you're not using too much compound. It is difficult to sand excess compound after it has dried.