I had my truck under coated about 6 months ago (don't ask what it looks like because I ain't seen it yet lol) and when I get home I plan on using a roll on liner to do that bottom body line. I did the latter to my first truck and it took about 8 hours from start to finish. It looks great. For the rust on the wheel wells I am going to sand it down, bondo if need be, and then put some bushwhacker fender flares on her.
2000 Dodge Ram 1500 Sport 4x4 - 5" lift, 36" tires, CIA, exhaust, CB radio, bed liner, undercoated, and more
2006 Dodge Ram 3500 Laramie 4x4 - trans temp, EGT, boost gauges, 5" exhaust, CIA, Fass 95GPH