Rust requires oxygen and moisture, and will not continue if they are not present. That's why it's important to protect the metal with primer, paint, undercoating, oil, etc. These products are super hard, yet flexible, and they completely seal off the metal, providing much more protection against rust. Some of them also claim to chemically convert the rust to some other form. If you want them to be completely effective, you should cover all of the metal, inside and out, and all edges, because the rust will continue on any exposed surfaces. I have used POR-15 with great results. I've heard that Rust Bullit is better, but I don't have any experience with it.