I came across this recently in the context of 'classic road bike' refurbishment. I had never heard of it before but it seems to be a genuinely useful technique.
Basically, you scrunch up some Aluminium Foil, wet it and use it as a light abrasive (making a slurry) on small patches of rust. Apparently a combination of the soft abrasive foil, and a redox reaction between the Al and the rust, causes the rust to be preferentially removed.
So far I have found the reported effectiveness to be true. Apparently it works extremely well on chromed steel. I don't think I have seen it mentioned here or on the uk.d-i-y Wiki, so thought I'd put it out there.
This is one of the better links I found on the topic:
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Cheers, Jon N