yr from karachi,,humid weather gets most cars there rusted. desel is not a good idea though. just keep underneath of car clean is enough, some cars are treated for rust some are not.. maybe you find some anti rust treatment for the metal underneath if nothing else at the most in exterme conditions leave a some polish underneath unwiped. unpainted area catches rust make sure you dont leave a dent unattended. you wont get rust on painted metal that's for sure, buy a product from "bullfrog" that makes diffirent antirust products,,,etc.
some companies make anti rust producta like softcarwax, anti rust lubricants,,,
go to yr local automotive store and look around there see what you can find,,