Bottom of Door Rust

The more you clean your car the longer the car will last before it rusts out. Consider a membership package at a nearby car wash. And if the door is only place its a problem, consider finding a used door at a junk yard and swapping them out.

 

Wet salt is more dangerous than dry

If you wax your car just before snow fly and then pick good weather times to do so mid winter it doesn’t hurt, sadly it also doesn’t help as much as you would want either and isn’t very helpful once it starts rusting

The old myth was washing too often actually speeds up rust from the car being wet during dry spells

 

Suspect the best way to prevent this would be building an undercarriage washing system in your driveway and an undercarriage drying system in your garage, as well as periodically have a new coat of undercoat paint applied. But obviously that's not very practical and once the rust gets started it's not so easy to stop.