Through different conversations with family members and friends, I have been told that it is good to apply a coat of car wax to the outside of the air conditioning unit frame. I assume that this is to maintain a protective seal and prevent corrosion. Has anyone done this before? I just bought a new Trane XR15 unit in April and I want to keep it in tip-top shape for as long as possible.
If you do this, make sure you don't get any wax on the coil! Doing so would hurt the unit's performance.
As others have said, this is totally a cosmetic thing. It won't help the life of the unit much. When the unit needs to be replaced, it will be because the coil or compressor is busted (probably 15yrs from now), and you've decided it isn't worth repairing. It's not going to need replacement because the frame has some surface rust.
The Trane unit has a special coat of paint that will make it look good for years and years, I doubt you will need to wax it.
This will decrease your capacity and efficiency
As stated http://www.ccacac.com/maintenance-tips/tips/maintenance-homeowners-can-do/ it talks about just weeds, and dirt much less a thick coat of wax.
Keep your outside unit clean and free of debris and plants. Grass clippings and vines can clog the outside coil and restrict the airflow. This will decrease your capacity and efficiency. Make sure that bushes and shrubs do not grow too close to the unit and restrict the airflow. The air must be able to escape freely from the discharge of the unit. Any restriction in this area can cause the hot air to be recirculated back through the unit. This will make the unit work much harder and reduce the efficiency and capacity.
Rinse the coil on the outside unit. This will clean the loose dirt, salt, and sand from the coil. Do not wash it with pressure, simply rinse it from the top down to let the dirt freely flow out. Washing it with pressure may pack the dirt into the center of the coil. Pressure should only be used from the inside out to push the dirt out the way it came in. Rinsing the coil should be done about once a month during the cooling season.