"Rust never sleeps" - you got that right!!! Sometimes I feel like I do nothing but chase corrosion. Painting the "fins" - you're talking about the little delecate strips of metal on the evaporator and surround the coils, right? Are those rusting? Aren't they made of aluminum? I don't think I'd put paint on those. You may interfere with the distrubtion of the "cold" and cause a total failure of the unit, or at the least, your coils mail begin freezing at which a total blockage of air flow will be created you won't get any air moving across the coils. Other than the metal box the unit is in, I'm not sure what's rusting. I've got 32-year-old evaporators that are still in operation and no rust....not even in my salty environment.
There are spraypaints out there that claim to stop rust. I deal with this issue on a grand scale, and I've tried everything. There is nothing you can paint over rust to stop it. You've got to first remove the rust or change the chemical properties of it. A dipping of a rusty part in phosphoric acid will do a good job of arresting the rest and changing its chemical make up. Naval jelly will do that too. Then, you can begin the task of refinishing/paintint the item. If you paint anything aluminum, you'll need to first apply a chemical to etch the surface, then a primer, or the paint won't stick.
When I paint my engine parts, I first strip it down to the iron or bronze and then spray it with a zinc chromium spraypaint primer (I think that's what it's called - it's neon lime green). Then I brush on Rustoleum enamel. I had a very nasty, rusty heat exchanger that I was going to replace until I learned how many thousands of dollars they cost. Instead, I used the process described above, and it sure is pretty two years later and not a hint of rust. I just love to run my hand over it when I pass by the front of that engine. Smooth, clean and still looks brand new. Now, I've become anal about anyone leaving dirty fingerprints on my engines.
This painting process isn't just for engines - it's for metal. I refinished my window frames in this fashion.
If the insides of your AC unit are rusting, you may want to do a little research to find a manufacturer that uses better quality materials. You'll pay more for it, but it will last longer - a tough lesson to fully learn and stick by, but I'm getting better at it. I have noticed that Chinese stainless steel is nothing like that made in the USA. There are too many inferior materials in Chinese metal. After picking up some of that stuff and watching it rust in a few weeks, I made it a point to only buy made in the USA metals..
