Its not the tarnhishing he wants to clean....?
'Gungy'
Accumulation of stuff on it.
I'd just take it all off, take apart the bridge and wipe it all down with a damp cloth, make sure its dried thoroughly and stick it all back on. If theres anything terribly stubborn I usually use WD40 (not on anything painted, though). That could be a gamble with the nickel but I dont think there're be a problem.