Primer question

I would treat it like bare metal. In the body shop we used to peel the paint off using 80 grit,then shoot epoxy. This left enough "tooth" for the bondo. You want your rust treatment to be in thie pitted rust. Be sure to work it in a bit.

If you dont leave the surface rough,you can be sure your paint wont stick for long.

Tell the truth,you really dont need that stuff. A good high in zinc primer should be enough.