Wiki suggests the following, although I wouldn't recommend it on stamps and/or covers:
Quote:
Foxed documents can be repaired, with greater or lesser success, using proprietary bleaches, dilute hydrogen peroxide or lasers. Each method risks side effects or damage to the paper or ink.
In reality, the minimal value of the above scanned covers (SCV $1) is only worth about 10% to 20% of that figure on the secondary market (less because of the foxing and considering they are uncacheted) -- so (in my opinion) it wouldn't be worth the exercise to try to "clean up the cover". Bottom line: It is what it is!
If you really want clean, cacheted specimens, they can be had for $1 or so on
ebay as in these examples (with no bids):
http://www.ebay.com/itm/US-U547-Cac...em2336e5e585http://www.ebay.com/itm/Scott-U549-...em20e2380d18