The county I worked for had a 300 gallon above ground diesel tank. It was originally outside exposed to the elements. I installed a concrete containment structure and covered it with a shed. Rust was not a problem, even after all those years exposed to rain, but every tank needs to be equipped with a filter and water separator, regardless. There were no rust or water contamination problems with diesel powered tractors, trucks and fire trucks filling from this tank.
I also installed a new 500 gallon gasoline above ground tank, that was state of the art at the time, and it was protected inside and had cathodic ground sinks attached.
www.agtvault.com/tanks.html 
