P21S is good stuff.

Acid-free Cleaners -- These are mild solutions of alkaline solvent, usually ethylene glycol, with a wetting agent. These solutions creep under the dirt and brake dust, loosening and lifting surface grime. Non-acidic cleaners usually require some surface agitation (brush or sponge) but are safer to use and will not etch the wheel's finish. P21S is the best known 100% acid-free wheel cleaner. Acid-free wheel cleaners range in strength from mild (P21S), to moderate (P21S Gel), to professional strength (Eimann Fabrik Hi-Intensity).

Acid-based Cleaners -- These are widely used by detailers, car dealers and car washes who need to clean wheels in the shortest possible time or with the least amount of effort. Acid-based cleaners are typically 2% solutions of oxalic, phosphoric, and hydrochloric acid. Eagle One All Finish Wheel Cleaner is an example of an acid-based cleaner. While acid-based cleaners pack the greatest cleaning punch they can easily etch the surface of your wheel if allowed to dry. Care must be taken not to use acid cleaners on wheels with pitted or chipped surfaces. The acid will migrate into any fissures and accentuate flaking and peeling of surface coatings.

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