Late to the party, ...
I use a Dremel for this work. My accessory tool is #512E, Finishing Abrasive Buffs...
Dremel 512E Link
Rather than using it to attack a wheel tread fully loaded with years of the ubiquitous black gunk, I'll first do as others have stated and use a metal tool to carve away chunks of the gunks. Then I'll apply a generous coating of alcohol, Goo-Gone, lacquer thinner, mineral spirits, kerosene,...I've tried them all, used them all at one time or another over my 60+ years in the hobby and they all loosen that crud, some better than others, but sufficient for the moment. I also use a Q-tip, wiping off the excess crud.
Then, over the last few years I've finished the cleaning with a buffing using the above Dremel accessory tool. IMHO?...Awesome!! Fast. Very conforming to the wheel tread/flange contour. Inexpensive yet long-lasting. No flying metal (magnetic or not) chards/bits. ( I still HIGHLY recommend using safety glasses for ANY Dremel work, however!) A favorite in my tools collection, for sure.
I only use these buffers on metal wheels. I suppose they'd clean plastic wheels quickly, too, but I have none on my equipment as per a list of common caveats thereof.
While this particular Dremel tool suffix, 'E', denotes its compatibility with their 402EZ Lock Mandrel, they will also attach to a standard 402 mandrel. I use both from time to time.
So, FWIW, there's another option.
KD