I have a couple of 10g kegs that have some very stubborn stains on the inside. I've used oxyclean, ecolox, scrubbed with hot soapy water, but nothing works. It's sort of a hard gummy substance that just won't let go.
Tough call. It's a hard brown substance that I think is on top of the stainless steel. I initially thought it might be rust or corrosion, but I think it's something stuck to the stainless steel surface.