Foxing On Paintings

[roseandlily]

does anyone know how I can remove foxing/ brown marks from a painting?

[Khandro]

I take it, it's a watercolour on paper, is that right?

[roseandlily]

Yes it is

[Khandro]

If it is any real value it would be best to take it to an expert restorer (natch!). A chemical which is used is 'Chloramine-T'
Also know n as "Tosylchloramide or N-chloro tosylamide, sodium salt, sold as chloramine-T, is a N-chlorinated and N-deprotonated sulfonamide used as a biocide and a mild disinfectant. It is a white powder that gives unstable solutions with water."
I am a professional artist and I had a batch of old (very expensive) unused hand-made paper which had developed foxing and I was able to remove it by soaking each sheet in a solution, but that isn't the same as a sheet with an artwork on it, obviously.
If you want to try to have a go yourself, make a solution and apply it to one spot only with a cotton bud on an unimportant part of the sheet, maybe close to the edge, leave it for 24 hours and see if it works.

[EDDIE51]

Very complicated, especially on a painting rather than on paper as Khandro's excellent answer.
http://periodfinebindings.typepad.com/removing_foxing_and_milde/
I would not even think about doing it myself!