This is actually a great question without a simple answer. Some restorers tumble all the parts including nuts & bolts, & sure they come out clean - but usually a coating (nickel, brass, etc) has been removed - so once it's all back together it's anyone's guess how long they'll stay looking nice. I think autosohl is the go as it (shouldn't) strip the coat off & should also leave some protection from the product itself. For larger pieces if badly tarnished or with some surface coating missing I think it's worth replating.