im from Sudbury Ontario , way up north lol, pur 15 is your best bet for paint because its designed to stick to rust/bare metal and is tough as nails, line xing your underbody would take alot of work as the whole underbody would have to be masked, well everything you dont want coated as well as the body of the truck and that stuff goes everywhere. As for undercoating up here we use a product called Krown or Rust check wich would be like spraying your whole truck with wd-40, it sounds messy but it works , it stps everything from seazing, lubes all of your cables and wirring, the other benefit is that the inside of your doors gets s[rayed wich lubes everything inside.