69 Dart Roof Rust

Are they rust holes or just surface rust? If they are holes, I hate to say it, but your screwed.
If its surface rust it should come off with sanding and then metal prep. If they are holes the correct way is to replace the metal with new metal.
You could get it media blasted, if the person knows what he's doing.
If you do nothing about it and carry on with the priming and painting, the rust will just come back. Maybe you could try a rust converter of some kind.
If it's holes and not all over you would need to weld in some patches. If the rust holes are every where maybe its time for a new roof. Just trying to help.
It really makes a difference if there are holes or not. If they are not holes, its not as bad.