I have been decorating our house, and having done more walls, ceilings
and skirtingboards than I care to think about, have now come to the
stair bannisters.

I don't want to repaint them, as they are in fairly good decorative
order. However, there are a few problem areas that could do with a
touch up. The problem is, I have no idea what the existing surface is.

It is black, and fairly glossy. However, it is neither water or spirit
based paint - when you rub the surface with a damp cloth fairly
persistently, it comes off! (Testing this has not helped the decorative
order, mind you...) It doesn't look like any sort of paint or varnish,
in fact.

The wood of the stairs has some tin-shaped stains on it (under the
carpet!) which look like they have come from a fairly watery substance:
the stain, whatever it is, has soaked into the surrounding wood rather
than just drying where it was.

Does anyone have any idea what this stain / wax / colouring / stuff is,
and where I might get some so that I can avoid a complete repaint?