Heres I would do it. Grab this product called " Rust- Mort " ,...usually you have to dab it in the area with a brush. In this case I would suggest somehow squirting it into the seam and moving across the seam evenly. I would do this several times. The product stops rust from continuing to spread if theres any you cannot see etc. Grind down the seam area at least an inch on each side of the seam. Allow the Rust - Mort to dry, inside as well. Next primer the area everywhere you can,..once the rust mort is dry,...I would even try to get some primer down the seam. Use Liquid Metal or Liquid Plastic ( same thing ) and use your spreader and treat it as if it were bondo. Sand it, shape it,...prime er up one or two more times, then paint away. A picture would help if ya can. Hope this helps.
