I have a 95 Ranger XLT, and don't have any rust in the cab corners, or rear fenders, but was wondering what I can do for preventitive maintenace, so I don't end up getting any in those problem areas?

 

Rusting is oxidation, the oxygen in the air absorbs electrons from the metal, fire is also oxidation but happens much faster, both rusting and fire give off heat, but you would be a long time roasting a marshmallow over a rusting piece if metal, lol
Oxidation rusting literally "burns up" the metal, just like a fire will burn up wood, if left to do its thing no metal remains

Water speeds up this transfer of electrons and if "salts" are added then it really speeds up

So any bare metal contact with oxygen causes rusting, this is why painting the metal prevents rusting, cuts off contact with oxygen, simple as that.
Painting over rust will slow it down but once it has started it will continue.
There are products like Navel Jelly that can use a chemical reaction to change the rusted metal into an inert material that won't react with oxygen, but just at the surface, if its still rusting under that surface then it will continue to rust.

Auto bodies rust from the back side usually, there is a cavity that WAS painted when new but moisture collects and paint comes off the metal so rust starts
Or paint is chipped on the outside and oxygen can get to the bare metal
Anyplace oxygen can come into contact with the metal "starts the fire"

Under coating works, but if rust has already started it just slows it down, but it DOES slow it down.
Adding ventilation may seem contrary to prevent oxygen contact but drying out a cavity slows down the rust, water/moisture is like gasoline on a wood fire when rust has started


Paint can't stick to metal very well, but Primer can, but Primer is not "tough" it deteriorates with sunlight and mosture
Paint can stick to Primer, and paint is "tough", resists most elements except physical removal, scrapes and chips, lol.