What to Do About Rust
A rusty air conditioner can be fixed, if you catch it in the early stages. With a degreasing solution, a nylon brush, and some sandpaper, you can even remove it yourself. Then, repaint the unit with a rust-proof primer and a layer of rust-proof paint, to prevent it from happening again.
HVAC - Evaporator Formicary Corrosion 1 anode per AC unit
evaporator coil
converts rust to a more stable oxide