AC condenser coil cleaner is exactly what you are looking for. Don't use it on polished aluminum though beacause it will errode the surface to a matt finish.

 

Where do you get something like that?

 

Check out Naval Jelly for aluminum, at Lowes or hardware stores. They also make a version for steel. It works well but if there's corrosion damage nothing will work for that other than sanding the damage off, even then the stain can be deep into the alum.