I have done this a couple of times on used horns that were covered in grime. Otherwise, I just let my horns oxidize. There is a German product called "Simichrome", a pinkish past that easily can be purchased in the US. It is very, very gentle on the brass and leaves a fairly rugged film that prevents tarnishing. The only downside to it is that if you don't get everything polished afterwards there will be a bit of whitish residue around the post. IMO, Brasso is way over the top unless you are stripping off lacquer.