Much of the trim on my fiberglass Boler trailer is aluminum, the window frames and belly band are extruded aluminum while the door hinges are cast aluminum.
With a little work and some common tools you can turn a dull rough cast hinge into a shiny custom part.
The original mill finish aluminum under a microscope actually has a surface that looks like small hairs, each of these hairs increase the exposed surface area and will oxidize causing the dull finish. When you use a power polisher with the correct cutting compound you actually remove these hairs making the surface glass smooth, this reduces the surface area and the reflection actually comes from the aluminum surface. Because the polished surface has less surface area it is less prone to oxidation and a simple wipe will maintain the surface.
Chemical polishes like Mothers will dissolve or actually convert the oxidation and remove it but the surface area remains the same which means it will oxidize faster and be harder to maintain. Using scotch pads or coarse abrasive pads leaves a uniform textured surface made up of small scratches in the same direction which, although looking good, will not directly reflect light, also the surface area is greater and therefore will oxidize faster.
The following video shows you how.