If you soaked it in acetone, some of the "crud" should loosen and maybe come off on its own. If that crown was mine, I would use a product called "Blue Ribbon" on it to loosen the crud, then VERY GENTLY, use a wooden toothpick to "encourage" more removal of the residue. "Blue Ribbon" leaves a slight film of protectant on the surface, so if intent was to get it slabbed, it would need to be removed with acetone. I like the protectant as it seems to inhibit further deterioration of the basic coin by rust or corrosion.
I don't do it and don't recommend it, but some "cons" will totally strip a coin like this, then recolor it in an attempt to pass it off as better than it really is. Just like anything else, if there is a buck to be made, someone will do it.