It’s usually the inside faces of the disc that rust like this with a very rusty patch where the pads have been sitting.Jack up onto axle stands and buzz the wheels by hand whilst holding some medium emery or a scotchbrite pad against face of disc. That should get rid of worst of it. You then need some hard braking out on the road...find somewhere with a long down hill run, where you can do some cadence braking. You’ll probably feel the patch under light braking for some time to come.
