No matter how new OR clean the concrete is, it will need a rough textured surface in order to achieve maximum epoxy adhesion. The rougher the better. There are many ways to etch (or roughen) the surface of the concrete: Acid Etching (using muriatic acid or our etch n' clean solution), Grinding the surface with a Floor Grinder/Scarifier, or Shot Blasting.
Though Acid Etching is the easiest, it can sometimes take multiple applications to sufficiently etch the concrete then you have to allow it to dry. More and More flooring contractors use a Floor Grinder with diamond plates to roughen up the concrete because it doesn't take as long as the acid prep. You can roughen the floor, sweep the debris away, and apply your epoxy in one day and it roughens the concrete much more effectively.
Also, if your coating concrete that has been sealed with a sealer, it is necessary to use a floor grinder to prepare the concrete. Using the grinder will open up the pores of the concrete to allow the epoxy to sink in, therefore achieving maximum adhesion.