Clean with mild abrasive, I used barkeepers friend because it was available.
Follow with a rinse and polish, I used NevR Dull.
Then Semichrome to buff to a shine and leave a protective coating behind.
but they were never high gloss and the layer of oxidation prevents oxygen
from getting to the metal and causing corrosion. Basically a layer of aluminum "rust" prevents the aluminum from rusting deeply. I cleaned mine with a small bristle brush and may have wiped the flat window portion with Semichrome but didn't try to achieve a high gloss polish.
You can buy belly band metal channel from Scamp
but it is a you bend to shape proposition.
A small bead of caulk along the top of the belly band helps stop the dirt streak run down from road grime collecting in belly band then being washed down by the rain. On a Scamp
the belly band is essentially cosmetic not a sealing band.
Although uncommon to leak at belly band I did have an honest to goodness leak at the belly band, the fiberglass used to join the top and bottom on the inside had a bubble in it that allowed a leak. I fixed the fiberglass, caulking the belly band wasn't the right fix. Or one that would work considering the amount of caulk applied by prev. owner hadn't done the job.