Just been discussed in another thread but I can't find it just now! [Could even have been you] This is due to the flux used to clean the joint during soldering. 'Wiping whilst hot' is apparently a shooting offence. If the acid flux is cleaned off it should not come back (hopefully). I would assume that warm water and a mild detergent would shift it. Don't know if you have to neutralise it to prevent it coming back from traces. If so, presumably some weak alkali solution (although I suppose that could cause other problems) :-(
Spray with something like WD40 to clear any remaining water based solvents?
Or just polish every day with Brasso :-)
Hopefully someone has a tried and tested cure.
Cheers Dave R