For surface rust treatment use a phosphoric acid based product. The acid reacts with the steel and forms a layer of iron phosphate, which is usually gray to black in appearance, depending on the metal's content. The brown bridge stuff is the same thing just with a different metal formula in the bridge. Either way, the phosphate seals the metal from outside moisture and resultant iron oxide formation (rust).

If rusted through you need to cut out all the non solid metal. Then find a buddy with a sand blaster to whack what is left. Make sure you do the underside as well as the bed. Once you have the rust off, hit it with the phosphoric acid compound and let it dry. If the compound is one of the ones that has a soap in it, just hose off and let dry. You should have a black metal base left. Then bondo, braze or whatever you wish to get it finished.

If yo anually undercoat the box with wd40, that will keep a lot of rust formation from starting.....

Had to do on both my T's. Works.