We have some old dark staind wainscoating in one of our bathrooms.
I would like to clean it up and put on a protective coat of poly.

I'm not sure what the old finish is, but I was thinking of doing the
following:
1. Cleaning it with alcahol or mineral spirits?
(I know if the old finish was shellac, then alcahol would tend to
dissolve it)
2. Cover it with a couple of applications of oil-based wipe-on satin
polyurethane

Is this a good idea?
If so, anything to add to my approach?
If not, any suggestions on a better approach?

If possible, I would like to avoid a major project of stripping off the
old finish and staining and starting from scratch...