I have successfully killed bad fogging on Slavich plates (without also killing the photosensitivity) by soaking them for exactly 60 seconds in a solution of;
20g Ferric EDTA and 10 g KBr per litre. (This solution keeps for years.) (Ferric EDTA is ethylenediaminetetraacetic acid ferric sodium salt)
What amazed me was that the photosensitivity of the plates did not drop noticeably afterwards without my needing to resensitise them in say 2% ascorbic acid (vitamin C) at pH ~6.