Which is better, rust proofing or undercoating?

Rust proofing and undercoating are related but slightly different, and which is better depends on what you want to protect. Undercoating typically refers to thick protective coating applied to underside of vehicle, underbody and wheel wells, to protect against stone chips, water, salt, and reduce noise, often rubberized or polyurethane. Rust proofing typically refers to thinner oil based or wax based treatment applied inside body cavities, doors, rocker panels, and seams where rust starts from inside, that creeps and displaces moisture, such as Krown, Fluid Film, or Corrosion Free. Many shops use terms interchangeably, but best protection is combination of both, undercoating for underbody physical barrier and rust proofing for internal cavities. For new car, hard undercoating for underbody plus cavity wax for inside panels is ideal long term. For old car or rust belt, oil based rust proofing that covers both underbody and cavities is better because it does not trap moisture over existing rust, whereas hard undercoating can trap moisture if applied over rust. So neither is universally better, rust proofing is better for internal hidden areas and existing vehicles, undercoating is better for underbody physical protection on new vehicles, and combined gives best coverage. Many experts recommend annual oil based rust proofing for salty climates as most effective.