I live in Key West where there is plenty of salt in the air. My car is left in a car port all the time when not in use. If I don't drive the car for a couple days the rotors always have a thin layer of rust over them?

What can I apply or do that will prevent this rust from appearing?

Isn't there stuff that you just spray on the rotors. I don't remember the name but almost certain that its just sold at the automotive store. I don't know from personal experience just remember reading a post about it long time ago. Someone had the same problem as you and there was a response of a product that you just spray on the rotors that cuts down on the rust?