The key to doing rust work is to be 1000% sure you have removed ALL of the rust and cleaned the area well. Rust is caused by exposing bare metal to oxygen and moisture. I would suggest using an angle die grinder (or something similar) to remove the rust. This will expose bare metal. Sand the area to be level with the surrounding finish. Make sure you clean it up very well. A tack cloth can work well after you've removed most of the debris. It couldn't hurt to rub the metal down with acetone (this may eat paint, though, so don't get it on the paint). Acetone readily evaporates in air. Once the area is dry, spray a bare metal primer on the metal. Make sure it's designed to be sprayed onto bare metal, as many primers are not. If it's designed for this, the container should say. Once the first layer has dried, I'd spray another layer or two over this. You can then lightly sand this and spray your base coat over it. Once you've done all of this, I'd highly recommend checking the entire panel for rust, and giving any rust you find the same treatment. I'd also highly recommend spraying rubberized coating onto the back of the panel, and any holes that are in it (that are supposed to be there). If you have enough, you might also consider spraying some onto the underside of the car. You're going to have to make sure any area you spray this onto is clean, though.