You'll get others chiming in here, but I usually finish my wood goblets with polyurethane... on the inside, I've used a Plexiglas solution as a safety measure... you can make a simple solution by dissolving a few pieces of plexi in acetone... just vary the amount of plexi to acetone to reach the viscosity you want in the solution... I like just about the consistency of sugar syrup. I paint 3 or 4 coats on the inside up to the rim, then finish the outside with my poly... about 6-8 coats.... you can do more and you may need to sand a little on the first 2 or 3 coats inside.