BIG,

It is readily available at most stores that sell A/C equipment. Check the concentration of Phosphoic acid to other ingredients, as you want the highest concentration you can get. If you can get a regular coil cleaner at an A/C dealer that is the best. Use a hudson sprayer to thoroughly coat the heads. The first time it will foam white is spots, just use a scrub brush to wash the part, rinse with water and keep spraying with acid until the entire part foams white, then rinse thoroughly and air dry. The cast aluminum will look like new. It will eat the aluminum but you have to let its set for hours to do it. It works wonders on the old style non clearcoated aluminum wheels. makes them look like new.

A lot of prducts have phospohric acid in them like oven cleaner and most purple all purpose cleaners.
John