One of two things is happening here. First, a chip in the paint (from a rock or whatever) that went through the CC the BC, and the primer layers leaving bare metal exposed to the elements. Rust forms under the paint layers and pushes out the paint forming a bubble. Second, the inside of the door is slowly rusting (maybe plugged drains, etc) to the point that it has eaten a pinhole in the metal. As the rust continues to build it pushes the layers of paint into a bubble. Both are fixable, the first easy and the second considerable harder. Given the age of the paint, my bet would be on the latter. Sorry, it is what it is. You could touch it up but it could actually make the problem worse as you expose more bare metal to make the repair, if you can even do that without making bigger and bigger holes in the door. I use some rust neutralization products but usually only in areas that I can't reach, or in areas where I have removed the rust and just want to seal any missed pin pricks. For a short term fix you can try and neutralize the back side of the panel, pop the bubble (hope no water comes out), lightly sand the rust down with sandpaper on the tip of a pencil eraser, mask off a tiny circle, apply rust stop, mask off a slightly bigger circle (maybe dime size), apply high build primer, sand this down very lightly, apply primer again (long sentence isn't it), sand to make even with existing paint, mask off (maybe quarter size), spray color to match, scuff up a larger area (maybe 3" x 3") and spray a few coats of CC. Not the proper way to do it but it could work. That old of paint will be tough to match and unless you detail the entire car the repair with stand out. Best of luck.
