The problem isn't the outside you show, but the inside lip towards the glass. You can treat the outside all you want, but it will still rust out from the inside. That's why the sealant material on a lot of these cars has a browish stain to it. They start rusting around the inside then progress to what you have. Best you can is do is clean all the rust off via grinding, sanding, etc. and treat with a sealant like POR15.