Sure it was rust and not exposed brass?
Wouldn't be the first time that has happened...
Cheers!
My stainless fermenter came with instructions to repassivate once a year or so.
If it is rusty, then it needs to be cleaned/passivated.
If it's not rusty (look for tiny specs of rust from welding/grinding/drilling spatter) then it's fine.
Did it say the best way to do it?
The instructions from Ss Brewtech are what @jimmyhugh wrote.
https://www.dropbox.com/s/c82ic4jtmjxewvb/cleaning instructions A4.pdf?oref=e
"Its a good idea to passivate your stainless steel equipment with an acid based solution. Filling the vessel with StarSan at 1oz per gallon of water or other acid passivation solution at the recommended strength and at a temperature of 70-80F for 20 minutes, followed by an air dry is all that is needed... To ensure the longest life for you stainless steel, you should passivate at least once a year or immediately after you may have done anything to scratch the surface."
my understanding from use is the fact that BKF is a fine abrasive powder and will actually remove the etching when wiped since it only on the very surface layer..You can use Starsan, which is mostly phosphoric acid, or Bar Keepers Friend, which is mainly oxalic acid.
You'll need 1oz of Starsan per gallon of water, or a thinly applied paste of oxalic acid. Let it sit for an hour or more, and rinse.
If you have etched volume marking on the kettle. don't use oxalic acid, as it will re-passivate the markings, making them hard to read.