This D20 was made to be paint fill-able and processed do your liking.

You MUST use a .2mm nozzle, and <=.12mm layer height to ensure numbers print legible. I used 6 walls each. 

Sand each face up from 80->4000 grit to ensure a smooth finish.

Use an acrylic paint and eraser to paint fill the numbers, then isopropyl alcohol + papertowel to wipe any excess paint. 

For the picked dice I used Rustoleum Metal spray paint and Vallejo Red.