Take a guess about the thick residue on this rim

Probly the gumbo they use to attach the pads to the steel backer; nothing touches that.
Second guess is the residue from old gasoline which also turns into a sticky mass. I think only acetone touches that, and it takes a really long time to soften it, during which the acetone flashes away.
I think I used MEK on that; Methyl-Ethyl-Ketone.