There are 3 main stages of rust on vehicles: Surface Rust, Scale, and Penetration, but first let's discuss exactly how rust is made.  Iron oxide is the technical term for rust, and it is formed when oxygen in the air reacts with the iron. Since vehicles are not typically built out of pure iron due to the inflexibility, tensile strength, and challenges with shaping it, materials such as carbon are added, causing the rust process to accelerate at the molecular level.

Surface Rust:  Paint nicks, cracks, and scratches are the first signs of rust. 

Scale Rust:  At this step, the metal strength of the vehicle is reduced. Dirt, grime, and salt that sticks to (or is trapped in) your vehicle begins to speed up the chemical reaction that causes rust because they act as electrolytes when dissolved in the water. 

Penetrating Rust: Holes in the steel are apparent at this stage.

While many cars are built with coatings and/or are dipped in anti-corrosion agents during the painting process, according to Popular Mechanics, "the road-facing side of the car is basically a sand-blasting cabinet at highway speeds, and those dips and coatings wear off over time."   

The Popular Mechanics article continues on to include ways to remove or reduce rust at each of the stages mentioned above.  To read more about it, click here.

The Driving website published an article on March 10, 2017 that explained why Spring is the best season to apply a rust-proofing application.  The article says,

"Fluctuating temperatures and a season of grime make the possibility of rust more likely than ever."
Centennial college professor, Chris Muir, told the website that, "It’s like condensation on a bathroom mirror. You’ve got cold steel that gets hit with a rush of heat and humidity from the ground, and it creeps into crannies and between panels really well."

Read the entire article at Driving's website or copy/paste this: http://driving.ca/auto-news/news/spring-not-fall-is-the-best-time-for-rustproofing?utm_source=dlvr.it&utm_medium=twitter