Something probably etched into the copper, in which case pickling in Starsan won't remove it. Still, it's just cosmetic and shouldn't be a problem.Thanks MaxStout - regarding the spots, even with Starsan they seem to remain. Even post-boil, they remain. I’ve seen other threads suggesting a longer soak in Starsan, so perhaps I’ll try that.
Make sure the copper "cleaner" has benzotriazole, this molecule will form a complex with copper ions causing a passivation layer.You might try a commercial copper cleaner compound.
^^^ ThisInstead of trying to sanitize the wort chiller prior to brewing, just immerse the chiller in the boiling wort for the final 15 mins of brewing to sanitize it. Then just rinse it off when you’re finished with Bree day.
This will not only sanitize the chiller, it will also shine the copper right up.
Fyi. Be careful because there is always a little water trapped inside. After you immerse, it may start spitting out on you. Or worse, into your beer.