Oh forgot to answer what I used, I am a hobby gunsmith in what little spare time I have these days and used firearm engravers enamel to do the fill with.
It comes in stick form semi-hard kinda like a big crayon and is very easy to apply and wipe away.
Todd
Sorry about the quality of the pic's but I suk when it comes to photog. The engraving is done in Gold even though it looks white somewhat in the pics.