I haven't made a cow horn handle like you have described but I have polished my share of it. Depending on how rough it is, start with course sandpaper or a file. Work it with finer and finer sandpaper to 600 grit then switch to rubbing compound on a rag and elbow grease or a cloth polishing wheel.
The cow horn knob handles I've made I filled them with fiber glass resin or a combination of fiber glass resin and wood dust.
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