I painted mine the way you first described. I first sprayed on the raised letter/number white color, followed by the black background color after allowing the white to dry thoroughly. I then wet sanded with 600 grit wet and dry and polished with rubbing compound and wax. If you can add a hardener to the raised portion and apply several coats you will lessen the chance of sand through and need for touch-up. Here is a picture of before and here is after .