First of all, it has to be made clear that rust does not come from paint. Rust is the substance formed when iron compounds corrode in the presence of oxygen and water. It is a mixture of iron oxides and hydroxides. An acid is then formed by dirt and dust mixing with the chemical reaction caused by Industrial Paint dampness and this acid eats away at the steel and iron, thus creating rust. Rusting is a common term for corrosion or oxidation, and usually refers to the corrosion of steel. To prevent rust, iron or steel should be painted as soon as possible. This should be done by removing any grease from the surface, then applying a metal primer and finally a gloss paint or one designed for steel or iron.
Paint used in cars can be acrylic-based or urethane-based. Urethane paints have gained favor in recent years because of its surface toughness and imperviousness to a lot of common chemicals. Acrylic paints, when spilled on by brake fluid for example, tend to bubble up, destroying the finish. This was a fairly common occurrence with inexperienced who worked on brake and/or hydraulic clutch systems and spilled brake fluid in the engine compartment. But before applying paint onto cars’ metal surfaces, it is highly important to ensure that the metal surfaces are free of oil, grease or moisture. Car manufacturers have spent tens of millions developing metal coating systems and technologies for their vehicles and this has manifested itself in the way newer cars are less prone to rust. Some factory systems put opposite electrical charges in a vehicle’s metal shell and the primer vat so that when the chassis is dipped into the vat, even crevices that cannot be reached by a spray will be coated with primer. Mazda has reportedly developed a paint system that employs a one-step baking and drying method, where three coating layers are applied in succession, meaning primer coat, base coat, and the clear coat while still wet.
All decent paint shops nowadays have paint temperature and particle-controlled paint booths and paint professionals ensure that the bare metal surfaces of a car to be painted are totally clean before the primer is laid on to the metal. This primer is then allowed to dry before the first coat of paint is sprayed on. The final step to painting a car is spraying on a clearcoat finish to protect the paint itself. As we said earlier, urethane paints are preferred nowadays because of the hard, plastic shell that envelops the car when it dries. Experienced painters know that dark colors, are more prone to finish failure because these colors get much hotter when out in the sun. The paint expands and contracts much more on a dark car than a lighter colored one. Over time, the cycle of heating and cooling can cause the layers of clear, color coat, and primer flake off the roofs, trunk lids, and hoods of cars, leaving the metal exposed to the moisture that will start the rusting process. A lot of rust happens from the inside out, however, specially in the door wells, bumper attachment points and wheelwells where road spray constantly bombards the car.