I have a sharpener which works wonders on old 45mm blades. I have a personal stash of "used" blades in their respective plastic cases. I wrote used across it with a black Sharpie... I keep using them.
I have found that a re-sharpened blade gets dull faster, and I don't know if it will hold a good edge after being sharpened more than once. I do know to use a tiny drop of sewing machine oil on the blade when sharpening. It helps with the sharpening.
Hope this helps somebody...
Anita