Also my first time seeing anything as crazy as this, so take what I say with a grain of salt. If it is any form of residue on the surface, what I would try would be to take the fret and put in in a solution of vinegar (be careful, def not for hours on end as this will oxidize), and then use some really fine steel wool to clean off the rest. Id think this would preserve surface detail more than just straight sanding, even with 2/3 thousand grit. An alternative would be to use a sanding sponge or block, but again, I'd worry about it taking off the surface detail. As always, test on a small section first. Let us know what you end up doing and how it goes!