There wasnt much real rust, on the ends of the rockers under the truck there was a tiny bit but i taught it a lesson with my angle grinder. The rest was just flaking clearcoat which im sure was going to become rust at once point, but i still sanded it.

 

spray on liner is a good idea but clean up the area with a grinder and then cut a section from the junkyard or a parts blazer and then patch the area then spray it so then the rust doesnt spread under the liner this makes it so u dont have to deal with paint and it will look good im doing it to mine cause my dad owns a bedliner company and the stuff works great!