Try a clear resin coating instead, like "Everbrite". Auto Enthusiasts and Motorcyclists use it to coat bare and polished cast sluminum wheels on their road/touring/show bikes and cars with supposedly great results. Best thing about it, is the clear resin coating is supposedly very easy to renew when it gets scratched up again and is "self leveling" so it is easy to apply.
Here's their link:
http://www.everbritecoatings.com/index2.html

Chombi