Mm, for me the canon on dross is pretty clear. Of course, canon only reaches as far as the boundaries on my own setting ;)
In order for the Glamour trapped in dross to be released, part of the item (and in many cases all of it) must be destroyed. A changeling must tear, crush, burn, devour, rip, or otherwise ruin the item permanently, unless only part of its Glamour can be released, in which case only part of the object is destroyed. Once all the dross has been released from an item, there can be no change to repair it. Ever.