Sorry if this is late...but I work as a detailer at a used car dealership, so I see a lot of used cars come in that had previous owners that hated to take care of the wheels If the stuff is really stubborn you will have to use really strong chemicals to get it off. At my shop I use a acid alluminum brightener. It works wonders, and can usually clean wheels up extremely fast. Today I was painting my rims black, and while I had the rims off the Jeep I decided to clean the backsides of the rims just cuz. I wouldnt be surprised if it was the first time they have been washed ever.
Started like this
Ended up like this
That was after only going over once. I really don't care about how my rims look like on the backside as no one ever sees them, but you can literally get any rim looking great with this stuff no matter how bad it looks now. The stuff I use is industrial strength, and we get it in bulk in like 10 gallon containers...so I'm not even sure if you'll be able to get it...but if you can, its worth looking into.