Step 1
Remove the rust off the air conditioner. You can do this with a chisel or a screwdriver, an air compressor, a sandblaster, a wire brush or chemicals that convert rust into nonreactive compounds.
Step 2
Scrape the surface with a metal brush, wash with detergent, then rinse and dry it.
Step 3
Paint the air conditioner with a rust-inhibiting primer and two coats of exterior paint to prevent it from rusting again. Apply the primer with a brush rather than spraying it on as this will get the paint deeper and inhibit rust from recurring.