It sounds like someone needs a winter top up!
Unfortunately, youve touched on an area of detailing that is challenging for professionals as well. Obviously, you could go crazy with acid
, that will get it off, but its far from safe. There are stories of brick acid being used on DW, with no negative effects (seamingly) but theres not a chance id use that on a customers car!
I would suggest something like Megs Wheel brightener, i have some of this in the unit, that i use from time to time on stubborn wheels. I would not suggest using this as a wheel cleaner you use every time you wash the car though, it is very strong, and should be used with caution!
Even after wheel brightner, i sometimes have to break the clay out!
its a tough job. good luck!