This is one of those things that I think a lot of us struggle with. Especially if we want to apply a clear coat.
Zep Commercial Tile & Terrazzo Cleaner is a concentrated, industrial-strength cleaner and degreaser designed for grout, ceramic, quarry and Mexican tile, concrete, and terrazzo floors.
Buy the clear coat epoxy kit and do what the instructions tell you to do. Anything else will void any hope of a warranty.
Usually people use an Muriatic Acid wash / etch before sealing or top coating. If you don't get it clean the top coat will never work.
You can also use a mechanical method like a floor sander to take the top material off if its too rough or dirty.
You can get pretty into the process and get some concrete stain and color it before sealing. They also make an acid stain that makes it look like marble with the veins and all.
Rinsed properly, it doesn’t leave a slippery film.