I know a couple of metal polishers. If the part is already perfect, they both recomend just using a regular application of any cheap 'pink' glass polish such as 'windolene'. That keeps the dirt & oxides at bay. Not so great if you ride daily or through a rotten winter though.
With the weather here in the UK being less than idea for bare alu, especially with salt on the roads in winter, I use a couple of coats of 'Harlywax' - it's a block of pure carnuba wax. Much cheaper, and much better than all those liquids that have about 1% wax dissolved in overpriced solvents. That forms a layer to hold crap away from the metal, and gives the best shine of any wax/polish I've seen.
If aluminium parts are tarnished, I bring it back to s shine with Meguiars NXT mextal polish, then re-wax. Honestly, I've used a lot of others, but this stuff's incredible. I use it on all 3 of my bikes, all of which have polished parts.
I wouldn't recommend using a clear-cote or laquer on hand polished parts. The finish is too fine for the coatings to bond well. Also, lots of Jap alloys are like swiss cheese. You'll get spiders of black corrosion creeping under the laquer and then it's a pig to strip and re-polish the part.