If you live by the ocean like us Floridians the polishing will not hold and I have never seen clear coated polish that actually retained the shine and didn't chip off of the extremely smooth surface.
On the other hand constant cleaning and use of the purple polish actually gives a nice result of the AL parts.
Painting is more forgiving especially if you have to hide a few imperfections.
Not that you would....just sayin'!
I have seen some really nice powder coating results and some pretty cool wrapping jobs.
Cheapest is still the polishing. I kind of like how Scotty got his parts painted though.
But that's just me.
Oh....and there is flat black paint.....
Nabs 
