>Whats the cheapest least invasive way
Spray it with kroil or pb blaster or some other such bolt breaker spray, let it sit a minute, wipe with a rag
, the fuzzy red rust will go away, the black will remain. oil lightly...(oil part is Important, with some nut breaker types, they have residual water and will flash rust after they dry)
If the rust is really crusty and flaky, soak more than once, **lightly** wipe with a scotchbright pad while wet with solution. Use circular pattern, so you dont make light/dark streaks. Be gentle, you can always do it again, you can never undo it ; P
Other folks probably have different methods..
Your mileage may vary...