Hello Arjan and Willem,
I can understand your point of view very well since I collect also artefacts from "primitive" cultures and I have a feeling for "dirt" patina.

But here we at the point: keris, special from Java and Peninsula are not "primitive" artefacts; this are cultural art objects. The wood of keris sheats is most of the time from wood with a beautiful grain and it would be a shame when it isn't possible to see it under all this "dirt-patina" and I agree with the others that you can't polish away the real wood patina. I haven't polished the sheat with steel-wool; only like Alan propound with cloth and car polish and then I have oiled the sheat with wood oil. And when you hold the keris in your hands you still can see the age of this piece and it have a very nice wood patina.
What you will do when you have a old Mandau sheat and the very fine rattan bindings are not to seen under a big coat from "dirt-patina"?
But sometimes I have a keris with both sort of patina and I conclude that I only polish the sheat a little bit and let it in this state, see the pictures. Or I only polish the front side of the sheat and let the back in "original" state.
Best regards,
Detlef