1) I use a small wire wheel on a cordless drill to clean everything I can.
The studs or the bolts' exterior threads, the hub itself, the hole in the wheel the hub goes into, the mating surfaces of the rotor and wheel.
It's amazing how much powder (corrosion) comes off.
2) What is pure zinc primer?
I was once given a spray can of zinc chromate (pale, pea green in color) to prime some aluminum.
An old airplane mechanic gave it to me.
I was told later that the stuff is no longer available.