We did it following way:
brush off with a steel brush the loose rust
apply a rust converter (we used tanik, http://www.tanik.be/TANIK-R-stop-rouille-14.html?lang=en , 1 liter for the whole truck)
apply a zink spray ( http://www.soudal.com/soudalweb/productDetail.aspx?w=1&p=52&ID=748 )
apply a...