"La's Totally Awesome" at Dollar General store. Cut it 50 percent with water. Spray on small area at a time, wipe with a sponge and wash off with water quickly before it dries.
Depending on how bad they are I use vinger and comet mixed together. That mixture even takes off waterspots and brightens the aluminum up. I used that mixture for many years on aluminum tank trailers that hualed bulk cement.
I used the Totally Awesome a couple of months ago on my trailer and it worked great and it is cheap got mine at the dollar tree a buck a QT bottle and does a killer job.