In our opinion you need to apply a surface coating. Great care needs to be taken around all edges to prevent water getting under the coating and turning black.
Something like AC lacquer (thinned with brushing thinners) or S&R's Danish Oil would do the job. If you do go the AC route all the old finish would need to be sanded off. You should be ok in applying S&R's Danish Oil over another but it is worth doing the coin test to check for compatability before proceeding over the entire surface.
It's also worth bearing in mind that you need to leave S&R Danish Oil at least 10 days to full cure.
Hope this helps
Ian
