In paper conservation literature, there are many ways and materials advised to be used to remove foxing or at least lighten its scars. Here i am not going to suggest what to use, but would like to mention the importance of a thorough knowledge of chemistry and physics too apart from the deep appreciation of the art work and its technique!
The conservator must know basics about oxidizing and reducing agents. Must know the chemical reaction and its byproducts before using it as a treatment.
Complications that might arise during treatment and how to deal with them, again needs a lot of knowledge and experience.
One of the problems that might arise while treating is formation of halo or tide-lines.
There is a lot to learn and practice for young conservators, before attempting treatment of foxing on original art on paper.