I haven't seen this mentioned while searching the forums, so I thought I would add to the collective knowledge
2001 Outback Wagon, 130,000 miles, started developing bubbled paint due to rust under the surface on the opposite corners of each rear fender (wheel well). The rust spots were right where the plastic bumper cover and the plastic trim piece near the rear door meet the sheetmetal. This was on both sides of the car in the rear only.
I finally decided to do something about it and took things apart and cleaned off the bubbled paint and rust using a wire wheel. As it is with rust, once I got further into the removal process, I discovered that the damage was much greater than I thought. Fairly large holes had developed near what appeared to be just surface rust. In addition, a large hole had developed in the body sheetmetal area just below the lower rear door (where the forward curve of the rear fender area transitions to the rocker panel section).
My brother was able to patch everything up for me using Bondo-Glass and POR-15 paint (Thanks Bro !)
I was still curious why this happened on a relatively late model car, especially since my wife has a 2000 Outback Wagon with no signs whatsoever of the same rust problem. Furthermore, I've kept an eye out for other Outback Wagons of the same vintage on the road and I have never seen another with the same rust problem.
After looking more carefully at the 2001, it appears that the rubber protective pieces that are press fit onto the curved edge of the rear fender (basically between the 2 plastic pieces I mentioned above), were not installed properly. The rubber pieces have an upper edge that is supposed to seal against the inner part of the fender to cause water to drain out properly. In my case, the rubber edge was creased downward in several locations. This seems to have caused water to get channeled under the rubber piece and run down and collect at the locations that rusted
Also, for some seemingly unrelated reason, I had some bubbled paint from rust under the forward edge of the radio antenna mount. Again, absolutely no signs of this on my wifes car that is a year older.
My car is in excellent mechanical condition. I am just hoping that the body will last long enough for me to get to at least 200,000 miles