As others have indicated, any coating over the metal will change its appearance. Also the more durable options are likely to cost more than those you can apply yourself.
Coating makes sense if you ride in all weathers or if you need to store your bike outside.
However if, like me, you are an unashamedly fair weather rider then leaving the metal bare (IMHO) is the best option.
Once buffed to the standard you want just apply a good quality non abrasive polish using a microfiber cloth . This will provide a protective film that keeps any oxidation at bay and only require the occasional polish to restore the lustre.