Wow.... Thats a lot of rust. I scrape the gunk off mine with some greased lightning a couple of times a year and I have never seen any rust at all underneath.

 

Seriously... just wire brush the big pieces but paint right over it with POR15. Google it and see...it's awesome stuff...forms a black shiny protective shield over the rust, blocks out air and moisture so it won't spread and it will be twice as strong a surface as before(in the case of floor panels or ones that have thinned out).

As soon as it's warm enoughmines going up on the hoist over the weekend and i'm painting the whole underside

 

You need to remove all loose scale before putting any kind of coating on. I recommend a scraper (one you don't mind sacrificing to the cause) then a wire brush/wheel treatment. You may need a variety of scrapers to get into all of the areas. Then a good rust converter before an form of a top coat. POR15 has some good products as does Eastwood.com.

 

rust is my best friendi get a nice breeze threw the pannels on my blazer and always up for a swim wile driveing