How to get rid of bad chrs?

[rikona]

Just did a backup of new box and got many errors using grsync. Almost
all were because of 'bad' chrs in the file name ["?: etc]. Some names
have more than 1 bad chr. Is there a way to safely remove just this set
of chrs in every file name within a folder/subfolders, and leave the
rest of the chrs in the file name intact? (safely = no chance of losing
a file :-)

Rik