If you live somewhere that is constantly warm and your air conditioner is relatively old it’s possible that your have rust on the grill of your outside condenser (air conditioner). There are a couple of things you can do to diminish or slow down the spreading of rust.
Applying a product that will slow down the oxidation process is the first option. What you need to do is clean the metal that contains the rust. Once the metal is clean and there are no loose rust particles apply rustoleum, or any other similar product to the metal. Know that the consistency of the rustoleum, or a similar product, will be thicker than paint and should be a dull gray silvery color. If any rust is left under the grill, regardless of which product you choose, rust will eventually come back.
Option #2, which is most likely the best option, is to do nothing. That’s right doing nothing may actually be better. If there is a considerable amount rust on your system, there’s a good chance it’s getting old and it may not be worth the hassle to get rid of the rust. Your energy, time, and money would be better spent researching and saving up for a new higher efficiency model.