Cleaning and Rust Proofing inside Frame Rails

I like the water based rust neutralizer/reformer. Like Rust Ender or Neutra-Rust. Can be applied with a pump spray bottle, paint brush, pushed with compressed air, whatever works. OK I haven't tried to get it inside frame rails. Imagining foam or other nap on a fish tape. I have used it inside doors, trunk lids, and such, applying in one place until it drips from other places. Little to no cleaning or prep work required.