Another take on that would be to spray the lettering with a can of black spray paint. You dont have to be real careful just make sure you get into all of the carved areas. Then sand the top surface and the paint will come up pretty easily and the black will remain in the carved area. Then go ahead and stain the whole thing. When you wipe up the stain, it will come right off the painted area. When finished and stain is dry, go ahead and brush Poly or Varnish on the whole area. If this is going to be an outdoor sign, I would use a marine varnish from a reputable marine store such as West Marine and not that Helmsman Spar Varnish which is a joke.
Tony B Retired woodworker, among other things.
"Strive for excellence and settle for completion" Tony B