This article is from the VW Technical FAQ, by with numerous contributions by Jan Vandenbrande others.
A: As far as I know, the jury is still out on this one.
Here are a couple of suggestions:
- The hard way: toothbrush, soap, simple green, lost of time, then wax
- High pressure power washer (rumored to be too harsh)
- Chemical products: Be very careful with these as some contain
extremely harsh chemicals (Hydrogen Fluoride compounds) causing
eventual discoloration or paint peeling from the plastic parts.
Some chemicals that do seem safer are (I have no personal experience):
o Meguires Mag wheel cleaner (approved and tested by BBS)
o P21S wheel cleaner (approved by Porsche, BMW & M-B)
Others that work, but I have no idea how harsh they are:
Westley's Wheel Magic, ArmorAll Quicksilver, Busch Wheel Cleaner,
Turtle Wax Wheel Cleaner for Mag Wheels
 
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