I am not sure of your exact application, but the method used by
watch and clock makers to highlight the engraving of dials is to
fill the engraving with either black engraving wax which is normally
shella based and is applied by warming the work and smearing the wax
over the surface. When hard the surplus material is the cut back
leaving the exposed surface. Being shellac based it can easily be
removed by using meths. Gerge daniels in his book on watchmaking
recomends Indian ink but the inks available here in Australia dont
seem to work, not for me at any rate. Hope this is of some help
Regards Steve