Depending on where you grew up, your reaction to the word “Rust” is either a shudder and glassy-eye stare reminiscent of PTSD sufferers, or a quizzical look as to what the fuss is all about.
To those who live in the midwest (especially the famous “rust belt” area from Boston and New York stretching west to Minneapolis and south to St Louis) or other states that salt the roads in the winter, it's a depressing fact of life that vehicle lives are usually limited more by rust than mechanical breakdowns. Rust on cars is such an assumed fact of life in these areas that a car considered to be in acceptable condition in Michigan would be almost considered totaled on sight in California.
By contrast, especially for those in the southwestern US, rust simply isn't a thing. Vehicles don't rust in the desert; compared to the midwest, they last effectively forever. A vehicle spending the majority of its time in Arizona or California will usually have its lifespan limited only by miles and mechanical repairs.
Rust is a cancer; once it's taken hold, it's *incredibly* difficult to treat completely. Usually the best you can do is mitigate to try and slow its spread before it gets too bad.
An ounce of prevention is worth a pound of cure.
A wrap-up of various rust-proofing/undercoating products common on the market: Real World Undercoating: Part 1 and Part 2
Short take-away; for application onto bare clean metal, Cosmoline (sold under many different brands) works extremely well. It dries hard and tough, and unlike “wet” rust prevention products it will not attract dirt/debris.
Lanolin-based products also work very well, however the coating is typically thicker and is not as durable or resistant to strikes from debris (such as gravel or other items kicked up by vehicle tires). It is however safe for accidental application onto rubber and plastic, whereas petroleum-based products may weaken or degrade such materials.