Maintaining wrought iron fences and gates in Arizona is simple. Don’t get any water on it.
Even raw, unpainted metal will take a very long time to rust to the point of structural damage unless it’s exposed to more than the little rain we get here. Landscape water from sprinklers, water pooling at the base of the fence posts and rinsing off of walks and decks causes more damage than the rains. Reclaimed water used on golf courses is so corrosive that typical industrial enamel is reduced to a peeling mess in a few weeks.
Areas using wrought iron for beauty or security should be designed with the iron not being sprayed with the landscape water. Fencing set in the ground should avoid being set in a depression that would hold water and preferably be placed where sprayed water won’t hit it. Iron fencing in grass is an invitation for failure.
In all situations it is important that metal receive at least two coats of paint, one coat of primer and one coat of finish. Most paints dry primarily by solvent evaporation. This leaves microscopic pin holes in the paint film that would allow moisture through and cause rust. With a second coat the chances of the pin holes lining up are every small reducing the likelihood of rust coming through the paint. This is the primary reason that two thin coats are better than one thick coat.
If water exposure is inevitable in your design situation, there are a couple of things that you can do to improve the longevity of the fencing or gates.
The most effective is to use galvanized metal as the zinc coating over the metal will protect the metal from the moisture.
Next would be upgrading the paint, using an epoxy primer and a urethane top coat. These synthetic products resist water and weather better that the standard enamel finishes. This is also the system recommended for fences where recycled water may come in contact with the metal.
An additional coat of paint will help protect the metal by giving another layer of protection.
Observation and maintenance. This is the key. Noticing deterioration early and taking corrective action. Once rust has gotten into the metal, especially inside hollow tube steel, there is no way to completely stop it.