Originally Posted by bassgod0dmw
Considering he has to cut new slots anyways, how would knowing what the previous fret size was be beneficial?
I would think that using frets with a larger tang/barbs than were previously installed would be the best option. That would eliminate the filler material completely, and you'd have new slots right into the wood.
It is beneficial to know the original tang size because of fretboard compression. When refretting a guitar neck that has a large forward bow but does not have a truss rod, techs and luthiers use a technique called compression fretting. Frets with larger tangs are installed in the bow of the neck. This forces the neck into a back bow. When the strings are installed the neck will pull into proper relief.
Fret tangs must be matched to the kerf. If a .022" saw is used, .022" is the proper fret tang size. If a larger tang is used the fingerboard will force a back bow. If a slimmer tang is used, the neck will bow forward.
As far as eliminating filler material goes, unless the neck was placed in a backnbow when the filler was installed (widening the slots) there is no more filler in the slot than the slot size allows.
In any event, this is not a do-it-yourself project. Most pros are not fond of re-cutting fret slots after being filled. Successful completion of this task will only be accomplished by the seriously handy. As always, if you think a file is something on your computer, consider changing a tire as working on a car, or have to look in kitchen drawers to find your tools, take this repair to a professional.