Polishing has to remove material down to the level of the bottom of the scratch. Thats a lot of material. The removal has to be even or you create a lens by the curvature of the surface.
I'd look into W/S replacement. In fact I am facing the same situation, and will be putting in a new W/S

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1982 300SD " Wotan" ..On the road as of Jan 8, 2007 with Historic Tags