I have a product called SEM Rust Seal, comes in many colors. I painted over the front wind deflector on the 240D, which was totally surface rusted, and only a few bubbles have come up. Works well. Also, a product that has worked really well has been Eureka FluidFilm, which Eberhard Weilke suggested. It doesn't get rid of already existing rust, but spray it on rust inhibited areas, and it really stops the spread. Also, it never dries, and it lubricates too. Truly great stuff.

POR15 is also good, a friend was using it on his Scirocco. The problem is it is very expensive, and it has a sort of expiration date, so you want to use it all and not buy more than needed. For a work truck, I'd go the rust sealing paint/fluidfilm route.

I also find that the rubberized asphalt undercoating materials actually worked well on the 240D...still holding up...

Those are my suggestions, but definitely use FluidFilm.