Just to clarify the original question- Red78445 has an instrument that (presumably) already has a finish on it and wants to enhance the finish.
Some finishes cant be applied over another type (i.e., applying boiled linseed oil over a cured shellac finish). If it was a specific type of finish, I would go with a newer application of the same on the prepared surface.
If its just a 'spit shine', I think some furniture or paste wax might be the best option. There are tons of suggestions available in various woodworking/luthier forums.