Did some work on my Highline this weekend.
My friend Take had a slow speed dremel with a very soft felt tip and we used chrome/brass/aluminium cleaner for that. Looks like new now.
You need something to handle oxidized aluminium.
Also re-did the bumpers with a re-finisher I bought in Germany. Looks like the cheap shoe shine containers with a foam brush on the top. Covers again scraped off paint with black color. Works great.
Just found a nice page which sells such products in USA.
Costs much more than the stuff I used, but seems to get the same result.

Above pic is from the link, not my one, mine looks better