It depends a bit on what is on it and how bad it is. Mud, manure and the like I will presoak with water or some light soap and get the big stuff off before I actually clean it with soap or degreaser. If it is greasy I may high pressure soap off the big stuff before going after it with a degreaser to clean it up. Either way the key is to get the big stuff before you even think about cleaning it otherwise you will be cleaning the same areas several times and just wasting your detergents.