How do you polish a cow/bull horn, to use it as a powder horn?

It depends on the surface of the horn. Some horns you can just take to the buffing wheel and they polish up nicely. If the horn has a coarse texture then treat it more like wood. Not seeing your horn, first hand sand it with 120 grit paper until the surface is smooth, then sand with 220-320 or whatever until you have removed all the scratches left by the 120 paper. Now wipe it down and put whatever finish on it you want. A clear spray finish works good, but so does just a varnish wiped on with a rag. You could also just get away with sanding it with the 120 grit and it will look nice after varnishing, if you want to be lazy about it, but the additional sanding should only take 5 or so minutes.

Thanks Mark
The horn came from a scrub bull and is a bit coarse.

Thanks again.

Rakes:
I'm no expert but I've made about 4 horns. I have used files, scrapers and sand paper (80 grit down to about 220) to get them smoothed down. patience is the key word. If you want a really high gloss, you need some sort of polishing method. I have used a buffing wheel and jewellers rouge to get a really high gloss but you could use polishing compound and a rag. Good luck.--Doc

Thanks Doc.

Bakes,

If the horn is coarse you can save some time by using a hand held sander on it first, just remember to always keep the sander moving to prevent any flat spots from happening.

Sand it down smooth, I use a powered sander get the rough horn down a ways, then use the screen style for drywall it don't load up. Far as polish is concerned, I have used fiberglass polish, but most buffing compounds for reshining a faded autos paint job works excellent.