dirt off of objects is fine. Fixing damage or hiding it is frowned upon by many collectors. You can search in the archives and find plenty of threads on this subject. Not everyone agrees, but some are very strong in their opinions that it should not be done. If you are removing burns, I wouldn't do it. Even if you are honest about altering the plaque, someone down the road may not be, or simply forget to pass on the information, and then it starts to get deceptive.

I do not collect plaques and I am not totally sure the same opinion holds true in that community, but I would hold off if it's anything more than wiping off some black material.

JMHO.

Michael Siskin