What is a trick to prevent fender-rust and rocker-rust that Ford's seem to be famous for ?
I am thinking about a 2021 Ford-Ranger. I do live in a place where they salt/sand the roads in winter, if that matters.

 

There is no trick that works

Rust is oxidation, which is the same chemical reaction that occurs in a Fire, so rust is a SLOOOOOOW fire, lol

Rust occurs when bare metal comes into contact with OXYGEN, the oxygen "burns the metal" consuming it just like wood is consumed in a fire
Oxygen is nasty stuff, but we need it to breath, lol

To prevent rust you simply don't allow bare metal to come into contact with oxygen, so prime and paint
Paint doesn't stick to metal very well, primer does
Primer is easily removed, scratched or deteriorated by light(UV)
Paint does stick to Primer and is tougher, resists scratching and light
So primer and then paint for best protection

Water is H2O, so has OXYGEN in it, so it will speed up rust/oxidation
Salts(any minerals) have "free ions" so will speed up Oxidation
Combination of water and salt.................faster oxidation IF............it has a way to get to the bare metal

So the key is no bare metal
Rockers and fenders are suppose to be DIPPED in primer and then Paint so inner surfaces are NOT bare metal
But sometimes they are just sprayed.......................so there can be some bare metal on the back sides and inside

Outside of moving to the Moon(no oxygen) the best way to prevent rust is to block metal's contact with oxygen
Undercoating is done for that reason