I know exactly where that rust is.
Get an angle grinder with a wire wheel, peel off that bubbling paint to the aft of the affected area, as in expose the extents of the cancer. Get all the loose scale off. Brush on a liberal amount of naval jelly. After that dries, coat the exterior with POR15. In the trunk well, if there is a hole, use some of that fibercloth then POR on top of that. If you wish, you could use the type of bondo that doesn't absorb water on the outside to fill, then order some paint to match from the Sherwin Williams auto paint place or someplace similiar. Just get a little bit, won't cost much. Get a friend with an airbrush to prime and paint. That's what I'd do anyway. Depending on how good you are that repair will last the rest of the life of the vehicle, will look presentable, and won't cost you too much time or money. Just be sure and POR the ***** out of the trunk well.
next step replace your rear window seal

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