I have an air conditioner unit about 500 ft from the ocean. The unit corroded after four years due to salt water/air. I have installed a new unit and want to spray paint the coil to slow down corrosion. What paint do you recommend? B. L.
That's a tough one. The problem is that air conditioner coils are usually copper tubes wrapped with aluminum fins. Under normal conditions, they last the life of the unit. Under salt-water spray and splash zone exposure, they fail prematurely, as you already know. Actually, you are lucky that they lasted four years. My company made cargo container cooling units that failed after one transoceanic trip. We applied a baked phenolic coating to the aluminum tubes before they were wrapped with the aluminum fins to solve the problem. Unfortunately, you can't do that.
Painting the coils will be more of a problem than coating selection. As I said above, the best-case scenario is to paint the copper before wrapping with the aluminum fins. Since you can't do that, your only hope is to try to paint them in place. Unfortunately, copper oxidizes rapidly and provided loose corrosion products. These are normally removed before coating, but you can't do that I suggest you power wash the coils with a detergent solution and then rinse with clear water. After the coils are dry, have someone come in and spray apply an epoxy primer. Be aware that painting the important areas, the copper tube/aluminum fin interface will be nearly impossible. I think the Faraday cage effect will hamper coating them electrostatically. The coils and fins will lose efficiency because the coating will act as thermal insulation. However, that is much better than having them corrode.