Here's the method I use to paint raised letter parts like valve covers and license plates.
Primer the part 1st, then block sand the letters/numbers to make sure they are flat.
Paint the color for the letters/numbers 1st, then paint a couple coats of clear paint.
Paint your top color, let it dry.
Wet sand the flat surface of the numbers/letters to remove the top color. If you accidently sand to the primer, touch up lightly with the letter/number paint color.
Re-clear coat the entire part and let it dry.
Yes it is time consuming, but the finished part will look amazing.