I advise against refacing and/or revoicing hammers yourself. If you want to do some regulation chores, there are some you can do that can be easily undone or readjusted as necessary. But if you file and reshape a hammer improperly, undoing the damage means taking even more material off, or even having to replace the hammer. Bear in mind too, there are some tuners who are not qualified to do that particular operation and who defer to other trained technicians. If you have a decent piano, I would leave that particular job to the pros.