welcome to the forum. the leaf springs will be rusty, no problem there. new ones will rust within a year or so. doesn't mean they're bad. undercarriage, though theres a lot to. is there any rotted spots, or wholes? if you spray over rust it will be back less than a year. if you sand it down, clean it up and try to do a great job, prime and paint, it will still come back eventually. i would clean it up, primer, paint, and use a spray undercoating.
i'm struggling with a way of preventing this myself
let me know if this helps