I use acrylic paint - my favorite are the heavy body artist's acrylic paints. If it is a wood pen, I apply the finish before engraving, that way the excess paint wipes off without sticking to the non-engraved wood. Acrylic blanks are easy, wipe on, wipe off.
The few I have had engraved I used the fabric paint from Michael's and really liked it. I turned the blank a few thousands undersized and put two coats of CA on then had the engraving done. When I got the blanks back I filled the engraving with the paint, let dry over night then sanded on the lathe until everything was level then coated with 10-15 coats of CA sanded and polished. The CA over then engraving made the engraving really pop.
The paint is really thick and the nozzle has a small hole making the filling easy, it also dries very fast.
When using rub n buff. How do you remove the extra stuff on the pen? Will it hurt antler? Been doing searches but no one seems to use it to fill engraving