How to prevent rust on a car
Why do cars rust?
Rust occurs when corrosive elements like salt and water (a mix found on nearly all icy roads in the winter) mix with oxygen in the air. The paint on your car actually helps to prevent rust by keeping the metal underneath from being exposed to this elements. Over time, paint will chip away and leave metal exposed, and then the rust will begin to spread. There are also parts underneath your car that are not painted, which is why the underbody will generally start to rust first.
Some corrosion cannot be prevented using paint or religiously washing off salty water. For instance, your exhaust system can corrode because the exhaust gases mix with water that condenses on the inside of the exhaust pipes during cold days and forms an acid that will eat through the metal in the pipes.
How to prevent rust on the outside of a car
The first thing we'll do is try to prevent the rust that you see on the outside of cars. This is the rust that forms because the paint chipped or was scratched away and metal was exposed to salt, water, and oxygen. There are two easy things you can do to prevent this from happening: wash you car regularly, and wax your car several times a year. A good wax job will protect the paint, which in turn protects the metal. Inspect your car regularly to see if the paint is scratched or chipped. For most modern cars the bumper is painted but also plastic, so you don't need to worry so much about a paint chip there, which are common especially on the front bumper.
If you do find a paint chip you can either take it to a shop that specializes in car paint, you can buy one of those scratch repair kits you see advertised on TV, or if you're really cheap you can just use nail polish on a small scratch in places that most people don't look. Remember to put a nice clear coat on top of it. Avoid sparkles.
How to prevent rust on the underbody on your car
To prevent rust from forming under your car on the exhaust pipes, gas tank, and other parts, you need to keep it cleaned regularly. You can do this at a lot of full service car washes, but you can also do it at home if you have one of those sprinkler systems that goes back and forth. Just set it underneath your car towards the middle and let 'er rip. Move your car back for forth to make sure everything gets cleaned off.
There is also rust preventative treatment that you can apply to iron on the underside of your car that will prevent rust from forming. Chances are you'll need a lift to get under there and apply the treatment to all the vulnerable areas.
Hwo to prevent rust inside your car
You need to wash the carpet in your car every few months, especially in the winter. The carpet traps salts from your shoes and will cause the floorboards to rust. You can use regular carpet shampoo inside your car and it'll help get rid of those salts.
Electronic rust prevention kits
Electronic rust prevention kits are absolute junk. Don't you think that if something like that really worked, everyone would be using one? I mean, for a couple hundred bucks, you can prevent rust from ever forming on your car! That's a great deal, isn't it? This is just the automotive version of snake oil.
You can't charge the metal in your car with some silly think that hooks up to your battery to prevent rust from forming. If there were, car manufacturers would add this as an option, because even though they want you to replace your car every few years, they also don't want one of their models to look like crap. Do you really think Ford likes it when one of their old Tauruses is seen driving around town with 50% of the body panels rotted away? That doesn't help sell Fords. You can't prevent this type of thing with a simple device, you've just got to be diligent about rust prevention.