I recently came across a great solution to rust-rot forming at the inner lips of hood, doors, and trunk lids -- that little line where the surface metal is bent under and welded.
After thoroughly removing surface scale and cleaning with a good cleaner/solvent, I applied two or three coats of Dow Corning silicone rubber coating. It brushes like a gray colored paint, dries rather quickly, and seals that ugly rusted area from further rot. The silicone rubber is rather expensive $7 plus per quart N but has many other uses around your BMW. so it's well worth the price and available at any good hardware store.
Author: Gary Westervelt