the easiest way I've seen to do it is make sure all oil & grease is removed from the area you want to paint, use a small brush to fill in the letters, wipe excess off with a cloth & let dry. the paint should stay in the grooves & you're done.
All I've had today is like 6 gummi bears & some scotch
1. Apply paint to letters. Do not be conservative. Apply more than you think you need to fill it in.
2. Let pain dry.
3. Spray some brake cleaner onto a small shop rag or paper towel and wipe away excess paint.
4. Do not clean with an ultrasonic cleaner. It will remove the paint.
NRA Life Member
What do I know? I'm only a professional.