Rust can absolutely ruin the metal on a car. It is best to prevent the rust from ever taking hold on your car by washing it frequently. I recommend washing at least once a week, especially in areas that use salt on the road during the cold months. All that salt and dirt and grime that builds up tends to hold the moisture in and that leaves a perfect environment for rust to begin. It often will set up in a place that has been dented or the paint has chipped. Exam your car often for small spots of rust. It is much easier to repair the rusted area when it is just a small spot.
That’s really the key. You want to repair the area while it is only on the surface. Once the rust travels deeper into the metal it becomes a much bigger and more expensive repair job. My work restoring vintage muscle cars taught me early on that rust can be lurking underneath a shoddy patch job that has been painted over. You can do a check for this by simply going around the car and knocking on the metal.
That can tell you, based on the clear metal sound or the thud of putty work used to hide rust, if the car is solid or not. It is a test I do before purchasing any car, whether it is only a year old or is one of the vintage muscle cars that I will be restoring. You should also inspect underneath the car and check the wheel wells as well. Both are prime locations for rust to begin. Never buy a car that has been in an accident, even if it was just a small accident. They car likely received some sort of damage to the paint that will be a prime target for rust. Even if the person selling the car claims to have had the car repaired by an expert. Don’t purchase it. Even with an expert doing the repair, it is very likely that rust will take hold. Take the time to check out any car before you purchase. It could save you thousands of dollars in costly rust repairs, in the long run.