One of the luthiers here (I think it might be Michael) has suggested using string of a suitable diameter with a very mild abrasive (such as that used to refresh car paint, e.g. T-Cut in the UK) to polish the slots in the nut - sliding the string too and fro in the nut slots. Do protect the head and fretboard from any residue though.
Last edited by JohnB on Thu May 10, 2018 8:49 pm, edited 1 time in total.
Hermanos Conde 1968, Stephen Frith 2007 "Guijoso"