On the French list, Carlos Gonzalez, luthier, says he always frets a
lute with the same material as its strings: gut with gut, nylgut with
nylgut, carbon with carbon, etc.
He first sand-papers the carbon. The difficulty is tightening the
fret sufficiently.
This does seem a logical way of going, as I imagine that carbon
strings must "bite" through gut frets at quite a pace.
Regards
Anthony
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