It is best to create a profile in the concrete, as this helps the stain to penetrate deeper into the surface. Remove all types of sealers and cures, and get rid of any paint, glue, or other coating that might be on the concrete. A floor polisher or buffer with black pads is a good choice, and some companies may use chemical surface preparation. It is important to test a number of areas with water, and if you see visible stains or the water beads up, there is something you must remove. If the water causes the concrete to go dark, then it is ready to be acid stained.