What you are looking for is a "Rust Check Rust Inhibitor", and not the coating or rust killer products. Those are paints that convert rust to metal, or stop it from spreading (somewhat). You don't want to use that!
You're looking for a Inhibitor, which more like the WD40 oil. I think the oilish product is redish in colour.
Spray that Windshield Area you just worked on. The area between the Rubber and the Glass (not the paint). This will slow down the rust that is under there.
At the base of the doors, there are two drain holes (front and back). You can stick the little straw/extender and spray some stuff in there. This will protect the door seams, which BMW's rust at.
Also, front of the hood, above the headlights area, there are some tight corners. E46 rust there. Spray there.
I would also spray the lips of the fender (back side).