Painting - base moulding prep

I have wood base mouldings in my bathroom and want to put a coat or two of new finish on them. Wood in the bathroom isn't usually a great idea but these mouldings are in good shape, not beat up or chipped. They are stained and all I need to do is put on some varnish (?) and they will look great. What prep do I need to do? Like, what is best to clean them? I washed them well when I washed the walls, which I just freshly painted. I believe they have varnish on them now and would like to stay with that I think (suggestions welcomed). Thanks for any advice/help.

As long as the surface is clean you shouldn't have any problems appling a new coat of poly/varnish. A light sanding wouldn't hurt.