I would examine the area around the bubble very carefully to decide if there is a specific reason taht rust popped out there. If you cannot find a specific reason it is best to assume that "like" areas around the bubble are ready to bllom as well, and to traet the area accordingly (remove the paint, poke around with an awl, cut out a section, etc.). Obviously, while you don't "want" to find more rust, it is better to find it and fix the entire area rather than fix one spot only to have another one an inch away pop out a few months later.

If, however, you are certain that you have only ONE bad spot, I would remove the paint/rust with a disk grinder with a disk or knotted wire brush making sure I had bare metal showing well beyond the hole. Then, enlarge the hole until you have full original thickness sheet metal all around the enlarged hole. At this point you have to make a decision based on the size of the hole you have created.

If it is relatively small (say, 1/4" or less), you could elect to strike the hole with a ball peen hammer to depress the rim and indent a circle of 3/4" or so 1/8" below the level of the surrounding metal. Then, using fiberglass reinforced filler (Cat's Hair or Tiger Hair), fill the area completely. Then sand below flush and dress with polyester glazing compound, seal and paint, blending the repair into the surrounding area.

If the hole you create is larger (which is probable since the bubble-hole is on the backside of a crater on the inside that radiates outward from the hole), I would cut a patch exactly the same shape and size as the hole, and retain it in the hole with a magnet across the patch and the panel. I would then tack mig-weld the patch in a couple of spots, then remove the magnet and complete the weld around the patch. Finally, using a disk grinder, level the weld flush, and finish with a small bit of glazing compound and paint as above. There are variations on this theme (like depressing the area and lap patching, or creating a flange around the hole, then laying the patch onto the flange and welding or bonding with epoxy, etc.), but they all stress a secure, proper fix-not a half baked "sell it quick" fix.