WD40 sorted that problem out for me, when it loses its moisture a bit I just spray some more on. Just ensure u wipe away turns black eventually if just sprayed on and left on the paintwork...means more washing
Mines in next week for its second lock replacememt as its started doing it again. Tape on the u bit does stop it, but you shouldn't have to do that. They are replacing the lock and the U bit this time.