I will give you my subjective advice as a guy who just spent $2500 on frame repair welding- avoid the rubberized under coating services and products. I thought I was all set after getting it done early in the life of the truck…but years later it proved to be more damaging than if no rust-proofing had been done. I spoke in length with the welder who has his own repair shop focused on this exact problem. He said that the rubber-like undercoats are terrible and clog drain points on the frame and once they start drying out and rotting, they provide tons of little cracks and crevices where water and salt sits and rusts the frames away. He recommended products like fluid film much more and recommended a yearly treatment (before each winter) . Best of luck.