Painting alum siding is definately an option to make dead alum come to life.
Painting requires several prep tests. When you run your hand over the siding does chalky residue come off on your hand? Normally this is an occurance.
If so, you need to react to that.
I would first completely power wash the entire building. Let it dry thoroughly.
Then seek out a Sherwin william's company and ask for their Super Paint Latex Exterior. Then ask about Emulsabond. This is an additive that you will add to the paint to counter the chalk. You can spray this or hand paint it on.
The emulsabond eliminates the need to prime it first and allows the paint to self level and counter the chalkiness.
It has worked for me many times in the past when I paint those old army quanset huts and other metal buildings.