In metal, I clean it with the appropriately lethal solvent.
After bathing in the solvent, I spray paint the part.
Then while bathed in a mist  toluene and ketones from the paint I 
immediately wipe off the paint with a rag.
POOF the numbers are darkened and it's pretty permanent.

In wood I just run a  sharp  pencil in the groove.

I have used a pantograph that I made using Matthias Wandel's model to 
carve numbers and filled 'em with layers of paint and metal flake.  
Darned thing works  great.