Corrosion of ceiling fan bodies & fan blades
The best thing to do is purchase the correct fan for the environment that it is going to be installed in. Fans that are made for the outdoor/wet/hot weather are going to cost more, but they are made for that type of environment, and they should come with a warranty. 

As for adding a protective coating to an existing fan, the only thing that I can think of would be to powder coat it, but that is going to cost you. If it can even be done, the shop would have to disassemble everything, powder coat, reassemble, test (hope that the blades are balanced) and even then that does not mean that the interior motor/wiring will not go bad do to the weather conditions.  

If it where me I would get a new fan for the environment.