Before anyone comments, you are probably going to have to post a photo.

Have you tried copper cleaner? https://www.amazon.com/Wrights-Copper-Brass-Cream-Cleaner/dp/B0083PEZ00 They also make a powder, which might work better on the black spots.

I know exactly what you have....I have 3....2 still in need of polising.

stripper.....as in paint remover works on those black burned on stains....then go to steel wook and barkeepers friend

Hm, that's interesting. I didn't know wood resins could eat through metal. There are indeed some holes in this thing, which is why I intended only to use it for tinder. Thx

Flitz metal polish?

Wood resains don't eat through copper......that is likely from lye soap and wear.

Yes they work well for kindling...or magazines or yarn.