Brass is a very reactive metal that oxidizes quickly, and brass finish is a protective coating that can be applied to a brass product after buffing that will protect it against oxidization. Brass is an alloy made mostly from copper and zinc, but other trace metals are often added to add strength or additional resistance to corrosion. When exposed to air, the molecules in brass can react with oxygen and break down into other substances, in a process called oxidization. It is oxidization that causes metals to rust or tarnish, so by adding a layer of finish over a brass appliance right after it has been buffed, you can protect it against corrosion. There are two main types of brass finish, shiny finishes and antique finishes. Which one you should use depends on your decorating style and how you want your brass appliance to look.
Shiny finishes are a more modern style that have gained popularity relatively recently. They give the brass a highly polished, extremely reflective surface that makes the metal almost look golden. Some popular types of shiny brass finishes are polished brass and polished chrome. This type of finish only uses a thin protective layer, so additional maintenance is required to keep it in the best possible condition. There are chemicals such as lacquers and physical vapor deposition products that cam be applied periodically to better maintain brass with a shiny style finish. Physical vapor deposition products work in much the same way as the actual finish, and are popular for outside appliances because they provide additional protection against scratching.
Antique, or "distressed," finishes are a popular choice for people who want their brass appliances to match older brass items in their house, or they want fewer reflective surfaces to give a room a more cozy look. They contain a chemical called ferric nitrate that oxidizes the brass in much the same way as the air would, but because the oxidization occurs under a layer of finish, it is limited and will stop after a certain amount of oxidation.
Two examples of popular antique style finishes are Antique Brass and Oil Rubbed Bronze. If you want to save money, you can make your own antique brass finish at home. Pure vinegar can be used in place of ferric nitrate, and it doesn't necessarily have to be mixed with the actual finish. To antique a brass appliance at home, clean it thoroughly, then put it in the oven for a few minutes under low heat. When it is moderately warm, but not too hot to touch, brush vinegar over it, then let it dry for a few minutes. You can keep adding layers of vinegar until the brass is as dark as you want it.
It is important to apply finish to any brass appliance promptly any time it is buffed. If you wait even a few days, the material will start to oxidize and proper plating or clear coating will be impossible. Make sure to research ahead of time what kind of brass finish to use and have it handy when you buff any brass appliance.