if its SOLID brass; just put 800 grit on a random orbit; then follow up with 1000, 1500 and then buff with buffing compound. PLEASE DO YOURSELF A FAVOR. Ask a Navy or Army guy in his 50s or 60s about keeping brass polished. Its seriously annoying and CONSTANT. it never ends. If you want the brass look, have it powdercoated with clear powdercoat.
If its not solid brass, doing the above procedure will ruin it