Niels Berkers should apply Hydrophane and leave it a day or two to
soften the leather residue before wire brushing it off.  Then, after
degreasing with some meths, a soak in Coca Cola or Heinz tomato ketchup
could be used to clean just the threaded wire rods.  Be careful that
these modern materials/methods don't cause irreversible damage though!

Patrick Handscombe
Wivenhoe, Essex, UK