It's also worth noting that even among art historians there is much debate as to whether or not restoration is an acceptable practice. The controversial restoration of the Sistine Chapel in the 70s and 80s is an excellent example; the restoration was heavily criticized and many frescoes were damaged and in some cases erased altogether during the process. Of course, not all forms of restorations are equally controversial (both in the art world and the card world), but I would actually wager that the reaction to alterations to a painting and to a baseball card would be pretty similar among experts/collectors in their respective fields.
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