Rust is an electrochemical process. It works on ferrous metals (iron and steel) from the inside out due to exposure of unprotected surfaces to air and moisture. The possibility of rust exists practically nationwide-anywhere humidity, industrial pollution or chemical salts are present, rust can form. In coastal areas, the problem is high humidity and salt air; in snowy areas, the problem is chemical salt (de-icer) used to keep the roads clear, and in industrial areas, sulphur dioxide is present in the air from industrial pollution and is changed to sulphuric acid when it rains. The rusting process is accelerated by high temperatures, especially in snowy areas, when vehicles are driven over slushy roads and then left overnight in a heated garage.
Automotive styling also can be a contributor to rust formation. Spot welding of panels creates small pockets that trap moisture and form an environment for rust formation. Fortunately, auto manufacturers have been working hard to increase the corrosion protection of their products. Galvanized sheet metal enjoys much wider use, along with the increased use of plastic and various rust retardant coatings. Manufacturers are also phasing out areas in automotive bodies where rust-forming moisture can collect.
To prevent rust, you must stop it before it gets started. On new vehicles, there are two ways to accomplish this.
First, the car should be treated with a commercial rustproofing compound. There are many different brands of franchised rustproofers, but most processes involve spraying a waxy "self-healing" compound under the chassis, inside rocker panels, inside doors and fender liners and similar places where rust is likely to form. Prices for a quality rustproofing job range from $100-$250, depending on the area, the brand name and the size of the vehicle.
Ideally, the vehicle should be rustproofed as soon as possible following the purchase. The surfaces of the car have begun to oxidize and deteriorate during shipping. In addition, the car may have sat on a dealer's lot or on a lot at the factory, and once the rust has progressed past the stage of light, powdery surface oxidation rustproofing is not likely to be worthwhile. Professional rustproofers feel that once rust has formed, rustproofing will simply seal in moisture already present. Most franchised rustproofing operations offer a 3-5 year warranty against rust-through, but will not support that warranty if the rustproofing is not applied within three months of the date of manufacture.
Undercoating should not be mistaken for rustproofing. Undercoating is a black, tar-like substance that is applied to the underside of a vehicle. Its basic function is to deaden noises that are transmitted from under the car. It simply cannot get into the crevices and seams where moisture tends to collect. In fact, it may clog up drainage holes and ventilation passages. Some undercoatings also tend to crack or peel with age and only create more moisture and corrosion attracting pockets.
The second thing you should do immediately after purchasing the car is apply a paint sealant. A sealant is a petroleum based product marketed under a wide variety of brand names. It has the same protective properties as a good wax, but bonds to the paint with a chemically inert layer that seals it from the air. If air can't get at the surface, oxidation cannot start.
The paint sealant kit consists of a base coat and a conditioning coat that should be applied every 6-8 months, depending on the manufacturer. The base coat must be applied before waxing, or the wax must first be removed.
Third, keep a garden hose handy for your car in winter. Use it a few times on nice days during the winter for underneath areas, and it will pay big dividends when spring arrives. Spraying under the fenders and other areas which even car washes don't reach will help remove road salt, dirt and other build-ups which help breed rust. Adjust the nozzle to a high-force spray. An old brush will help break up residue, permitting it to be washed away more easily.
It's a somewhat messy job, but worth it in the long run because rust often starts in those hidden areas.
At the same time, wash grime off the door sills and, more importantly, the under portions of the doors, plus the tailgate if you have a station wagon. Applying a coat of wax to those areas at least once before and once during winter will help fend off rust.
When applying the wax to the under parts of the doors, you will note small drain holes. These holes often are plugged with undercoating or dirt. Make sure they are cleaned out to prevent water build-up inside the doors. A small punch or penknife will do the job.
Water from the high-pressure sprays in car washes sometimes can get into the housings for parking and taillights, so take a close look. If they contain water merely loosen the retaining screws and the water should run out.
One thing you have to remember about rust: even if you grind away all the rusted metal in a panel, and repair the area with any of the kits available, eventually the rust will return. There are two reasons for this. One, rust is a chemical reaction that causes pressure under the repair from the inside out. That's how the blisters form. Two, the back side of the panel (and the repair) is wide open to moisture, and unpainted body filler acts like a sponge. That's why the best solution to rust problems is to remove the rusted panel and install a new one or have the rusted area cut out and a new piece of sheet metal welded in its place. The trouble with welding is the expense; sometimes it will cost more than the car is worth.
One of the better solutions to do-it-yourself rust repair is the process using a fiberglass cloth repair kit. This will give a strong repair that resists cracking and moisture and is relatively easy to use. It can be used on large or small holes and also can be applied over contoured surfaces.