Brownell's sell Lacquer-Stik. This is a large stick of material that is rubbed into the lettering, allowed to dry for a few minutes, and the excess rempved with a solvent. This is what machinest and gunsmiths use to fill lettering.
It comes in red, white, blue, gold, silver, and black.
There are a number of things that will work, but this works best, and lasts about forever.