Engraving

This may be a bit off subject but hopefully someone will know. I'm
planning on using my CNC to engrave brass and also wood. The brass
won't be engraver's brass - that is it won't be film coated - but
will be straight unfinished brass. I plan on paint filling the
engraving by hand. Will paint stick to unprimered clean brass? I
can't seem to find anything about that. It just seems to me that its
too difficult to align tiny bits of paint over primer without either
having parts of the primer show or having the paint appear
differently where paint covers bare metal and where it covers primer.

The same question applies to hand color engraving on wood. I think
it might be easier here because one could just seal the whole piece
of wood after engraving and then color it. How is this stuff
normally done? It doesn't seem like it should be such a big deal but
I can't seem to figure out a way where it isn't.

Thanks, Eric