I have an 18 TRD off road with 11K on the clock. The day after I bought the truck new, I wiped the frame and underbody down with Prep-sol, removed all heat shields and spare tire, lowered the gas tank, masked the sides of the truck with newspaper and painters tape and undercoated EVERYTHING that was frame or sheet metal under the truck. I let it sit for two days jacked up with the tires off and the gas tank lowered and let the undercoating dry (July in NJ, HOT).
11K miles later, no chips in the undercoating and zero rust on the frame or underbody. Unlike the wax coating, I don't have to apply it every year.