I had the same issue and learn from my mistakes... do not use anything to clean the inside that is metal!!! I used a wire brush to remove the slag! BAD IDEA!
In retrospect it embedded small particles of the brush in the SS as it cleaned and left more metal to oxidize and rust... Use coarse scotch brite pads on a air angle grinder. If that doesn't do it use sand paper but only if you have too (you shouldn't). Once all the slag is cleaned then progress to finer scotch brite pads and you'll have a pretty nice finish.
Finally, get some barkeepers friend and mix it up into a thick paste. I then scrubbed it in using a fine scotch brite. I let is sit over night to give the Oxalic Acid time to re-passivate the SS. Then rinse well and wipe with a wash cloth until it's all gone. Then rinse one more time for good measure!
I haven't had any rusting in my keggles since I did this! It is time consuming and a PITA but it's better than wasting good keggles!
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