I must have polished perhaps hundreds of parts by hand over the years, due to something of an obsession with having Campag/Royce type finish on all components
One thing I've learned is that 6000 series aluminium seems a lot less prone to corrosion - so much so that some parts are sold un-anodised, yet are able to retain a polish and resist oxidation for years; think Campag and White Industry hubs, some rims, Victory and Triomphe mech's and shifters, certain Paul Component parts etc.
Unfortunately '6000' alum' lacks the durability and fatigue resistance needed, so is never (as far as I'm aware) used for quality cranksets.
All the cranks I've polished have at varying speeds started to corrode. On an MTB in offroad conditions this can begin to happen immediately. I've managed to keep one Campag Centaur crank looking respectable by regular attention and only light use, but another set is now so deeply corroded that even aggressive re-polishing would be ineffectual. Anodizing is used for a reason!
Interestingly, several companies refused to engrave or laser etch their logos bitd, believing that it would lead to corrosion, or could cause stress risers. (I'm looking at a black 3TTT stem at the moment - it's in mint condition apart from the etched logo which is all corroded and furred up through the anodising.)
There are apparently several surface treatments/waxes available that work well at preventing oxidation though. The car boys are the experts here and seem to have success with their beloved alloys. A chap on this forum was enthusiastically recommending something a while ago, I'll post a link if I can find the thread.
edit. here 'tis: Poorboys wheel wax ...see 'lewis1641' comment at bottom of thread: viewtopic.php?f=7&t=197564