Do you mean keeping a bright polished finish rather than a good machined finish, ie one free of tool marks?

A problem and virtue of Aluminium is it reacts quickly with Oxygen in air. The reaction creates a thin surface oxide layer that stops further action, so the metal is self-protecting, which is good. Unfortunately the oxide layer is dull grey and tends to spoil the appearance. For decorative purposes Aluminium is often anodised, where a simple electrical process thickens the oxide layer and embeds coloured dye in it.

Hard to keep a mirror finish on Aluminium. As far as I know, it can only be done by immediately varnishing the polished metal with a clear lacquer. Or, in the case of mirrors, air is kept away from the shiny surface by the glass.

Dave

If a turned piece, the a fine 'Scotchbright' type material used while inthe lathe polishes a treat. Otherwise the same with elbow grease if flat /irregular. Agree about oxidation though, so clear lacquer may be your only option to retain the finish in the long term.

Very highly polished aluminium will keep a shine for several years if kept clean, even though it will develop an oxide layer. With a well polished surface this is very thin and barely reduces the reflectance. Think aluminised telescope mirrors (you can get them overcoated, but this is often not done as it costs extra).

Results do vary according to grade and an overcoat of polish should help preserve the shine under real world conditions.

As random example, our back door handle lost most of its anodising years ago, and the worn parts are mirror bright.

Neil