im pretty sure in high school science we learnt about this, but i could be mixing up two methods (i havent read articles BTW), you need a sacraficial piece of metal for cathodic protection which will rust more readily than the car, it apparently works but you need no rust in protected metal, which pretty much rules all of us out!

pretty much all my rust was caused by clumped mud holding moisture to the metal and it wasnt a rally car when i got it, just a lady driven road car which i suspect never had a decent wash in its life, make sure you clean out the wheel arches and in the back corners under the car because a lot of crap builds up in there and rusts the boot floor and bottom of rear quarters, as for the sills i havent figured out why they rust so much, poor drainage maybe?