Cleaning is a three step process.
1) Use a heavy duty solvent cleaner that can cut through tar, grease, adhesive, etc. This can be Xylol (common brand name of xylene), most adhesive removers (we use Rapid Remover) work well. This step can be skipped if there is no heavy soil present. When prepping a vehicle for a wrap, even if the vehicle looks spotless, we will still hit he wheel well areas and rocker panels, as there is always tough dirt in those spots.
2) Use a detergent cleaner to remove any remaining light soil. Any sort of a soap will work well. 3M PrepSol works well for harder to remove dirt. Regular old dish soap works too. This step can be substituted with a car wash.
3) The final wipe should be done with 70% Isopropyl Alcohol. If you're getting dirt on your towels, you probably don't have a clean enough surface for a final wipe yet.
This is the *correct* way to clean & prep a vehicle. Most shops don't do this..