I'd wire brush it the best you can, prime the bad areas, spray a very light coat of rubberized undercoating on the entire chassis and then spray everything you can with a few cans of Fluid Film every fall. I live in a nasty rust belt and have done this since purchasing my 2011 SEL brand new and she still looks great underneath. I have gotten some rust along that lower rocker seam because it's really difficult to spray between the metal that's spot welded there. Also, it's a good idea to spray some rustproofing inside and outside of all the bottom door, hood and tailgate seams. I use a product called Amber Rustproofing. It doesn't take much time for annual rust prevention maintenance and it's definitely worth it if you live in a salt belt.