I would go farther than just a hand held wire brush to get it to bare metal. There are different types of wheels for a drill or angle grinder. The best primer is an epoxy or urethane, but Rustoleum auto primer will work fine. I don't remember what it's called, but there are rust neutralizers you spread on, then prime. I wouldn't apply Flex Seal or Plasti-Dip until you prime, and I wouldn't use it at all, if not applied properly, it seems like a place where moisture could collect. I used to live in MN so I've been cleaning up rusty cars since the 70's when you could hear the rust forming. I now live in AZ so its dried out components that are the problem. Good luck and keep us posted.