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I have heard of people trying to breathe life back into an old mainspring but one thing you must remember is no matter how you treat it you will never be able to undo tiny stress fractures. You could try this and the watch could run fine again for a year or so but do you want to take the chance of that spring breaking after all the hard work you have put into servicing/repairing it?
It can be frustrating when you are looking for odd size springs but if you look long enough you should find something suitable. If it was a customers watch the only thing you can do is explain that you cant find a substitute and you can put the original spring back in but you cant offer any guarantee against it breaking.
One of my earliest lessons in horology was not to buy used or old stock mainsprings, it is a false economy and to prove the point (before I was given that advice) I bought a box of old stock mainsprings for alarm clocks. 3 of them broke as soon as I took them out of their retaining rings before I managed to get one out that didnt break. Thinking back, that should have told me something but I still went on to put one back in a small alarm clock. So far it has been fine but I certainly wouldnt want to guarantee that clock against spring breakage and I would definately not put one in a customers clock.
If this metalurgist was a horologist as well and had performed this process at least half a dozen times and never had a problem with it then I would certainly try it but only on my own stuff.