ChrispenEvan said:

elbow grease and stainless scourer. it is carbon, tough stuff.

but I’ll be interested to see if any of the methods here work.

FWIW (and this is not helpful at all) I’ve used piranha solution, a mixture of concentrated sulfuric acid and hydrogen peroxide, but this is an extremely dangerous chemical. You need full ppe, including breathing apparatus, but it’s one of the few chemicals that’ll dissolve elemental carbon.

I don’t recommend it, though; it’s too dangerous.