I've researched this a lot lately, and purchased numerous pieces of used aluminium trim to experiment but I haven't got a as far as actually restoring the trim yet.
As I understand it, the problem is that the parts are anodised and the anodised layer has effectively failed, allowing oxidisation to occur beyond this protective surface. Anodising is supposed to prevent the aluminium from oxidisation by giving it an impenetrable 'pre-oxidised' layer. The milky blemishes are the result of unwanted oxidisation within the anodised surface and no amount of polishing or buffing will remove them (unless you polish very hard/abrasively and remove the anodisation layer).
My conclusions so far is that there are 3 options:
- Replace with new.
- Strip and re-anodise.
- Strip the anodised layer and polish.
#1 is likely to be very expensive, if the parts are even available. I haven't looked into this option much though to be honest, so I would be interested if anyone knows the cost and availability of the various 'chrome' parts.
#2 is probably the best option, though I have struggled to find an anodiser who will strip and re-anodise the parts. Most aren't interested and seem to be involved in anodising new parts or anodising on an industrial scale. I still haven't given up on this option though, if anyone knows of anodiser who will do such work? It's also possible to re-anodise the parts yourself, if you're feeling ambitious.
#3 is an option that I originally dismissed but recently started to reconsider. The problem with this method is that the aluminium will quickly re-oxidise without any protective surface, requiring frequently polishing, and clear-coat/lacquers generally don't stick well to highly polished metal surfaces.
I recently came across this stuff however, which claims to be able to adhere to polished metal surfaces:
http://www.frost.co.uk/por15-glisten...art-946ml.html
http://www.por15.com/Glisten-PC-High...oat_ep_75.html
The idea is to remove the existing anodised layer using a caustic soda solution (or Lye, if you're American) like this:
https://www.youtube.com/watch?v=4BxFn0111Bk
... then polish the bare aluminium and use POR15 2K (aka Glisten PC) to protect the finish.