In reply to Stefan Jacobsen:
Petzl sharpener is good for fixing minor burrs and flats when on a trip, but the filing angle are to aggressive for repeated use, since only the tip of the tooth are filed down.
To reestablish sharpness and angels of a tooth, the cheapest and fastest way is by using hand files.
I've used the below procedure, files and methods for years, which work well:
Post edited at 15:16