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I prefer rust proofing to under coating. The former was made to seep into lap joints in sheet metal. This is usually the first place that rust starts. If you look on flat surfaces these areas are 9/10 free of rust. If anything its minor surface rust. My experience has found that undercoating seals only for a short period of time. Cracks form and let water in and then rust. The sealed cavity stays wet, something that does not happen on flat surfaces. If you want to deaden the sound to make the ride more quiet then 3M makes a product for this that you can spray inside door panels, fenders, and in the cab area. Maybe someone else has more info or experience using other products. You need to be specific though on what you are trying to achieve. Are you wanting to increase the life of undercarriage components or have a quieter ride? If you want both then you need (in my opinion) to use two different products and thus different methods of application.
Line-X is suposed to be the best spray on bedliner created, and is not suposed to crack, fade, etc...
Line-X is a bed liner that is suposed to stand up to shovels, rocks, and anything else you can throw at it, and not is not widely used as an under coating because of its price.
I would be intrested in hearing from some of you who use Line-X as a bed liner and your thoughts on how it would stand up as an under coating.
Since you can get it in a few colors, and can even have it tinted to match the color of your truck, I think it may be a good alternative to the cheap useless spray on undercoatings.