Before applying a concrete sealer or water repellent it is typically needed to prep the surface first. Prepping consists of cleaning with water and using a chemical cleaner that is specific to the stain you are trying to remove. Cleaners for concrete and brick pavers come in many different formulas.
Lets look at the main reasons to clean and prep your concrete:
General Dirt and Grime Cleaning
When cleaning to remove dirt and grime you may just need water and a pressure washer. If the stains are stubborn, then a general all purpose concrete cleaner or even bleach would help with the washing.