Gotta admit, I'm [like many others on here] sitting here scratching my head wondering what your doing and why. The 'rust' you are having sleepless nights over is nothing. Personally I think you have made a mountain of work for yourself. Close the boot, open the bonnet and do work there.....thats where the go fast stuff lives!!!!
If you must, ,,,for that 'rust' under the car, coat it with Septone converter, then use an undertray primer/bitumen type paint and forget it.
https://www.repco.com.au/en/car-care-panel/paint/underbody/c/1862917956
https://www.supercheapauto.com.au/p/septone-septonechassis-black---1-litre/1411.html