Anyway, when I look at some of these old stamps, I think there is just one thing I can do to improve them. I never repair stamps but I do wash them. I think that is legitimate because it returns the stamp closer to its original state before collectors got hold of it. More or less, you can get back to the state a stamp was in at the moment the first dealer or collector washed it off an envelope or a document. Unused stamps pose a separate problem: if there is no gum left, then it is reasonable to wash them. But if there is still some gum, it is probably important to retain it. So washing is ruled out.
Of course, some times you discover more Bad News: a thin in the stamp hidden by a hinge, for example. Or a stamp which breaks into two when the hinges come loose. But most of the time you end up with a stamp with better colour, a more even surface and no accumulation of stuff on the back which does not belong to the stamp and should never have been there in the first place.