CLT copper problems

[BrewerMel]

Hi all,
I've got a cold liquor tank that, while it used to be copper, is now black and apparently has been for over 8 years. Seems the two brewers previous to me didn't feel the need to clean it and thus the black tank.
Any ideas on the best way to clean and then keep this tank? I've used an acid wash (and a ton of scrubbing) on it and it has taken quite a bit of the black off but I'm still having issues with it. There seems to be a yellowish residue that continues to react with the copper despite repeated attempts.
Does anyone have any experience with something like this? Any help would be GREATLY appreciated! My arms are getting tired.
Cheers!
Mel

[MikeRoy]

I use a product called Barkeeper's Friend, I apply this to a wet bar towel and scrub a small area and rinse off. When I'm finished with a tank I polish it with Brasso or Noxon 7.

[gitchegumee]

There's an extensive thread on copper cleaning somewhere on this board. Take a look an ye shall find. Good luck!