If you can choose between methods of car rust proofing, you should choose a car that has had the rust proofing applied in the factory. If you are buying a new car, this information should be readily available to you. If a dealer tries to apply rust proofing to an older car, he may not do a high quality job. If this is the case, the rust proofing may or may not work, and it may even damage the paint finish on the vehicle. If you can choose between the two, the factory applied rust proofing will be a much better option for your car.