That is typical "Subie" rust. The quarters rust right at the shelf where the quarter meets the bumper cover. Rust coming from the backside like this will most likely necessitate replacement of some inner structure as well. (at least to do it right, it will) as far as the outer panel goes, you don't need to invest in a welder. Go to your local Harbor freight store and buy a flanging tool.




Cut out the rusted area (rust is like cancer! The only way to stop it is to CUT IT AWAY!) and flange the edges around the hole, fashion a new piece of sheetmetal and "Glue" it back in with "Lord, fusor" product. The panel will be flush, completely sealed from moisture leeching through the repair into your plastic filler skim coat on the outer surface.
The upside to this method is that you will not be burning away any of the factory E-coat on the back of the sheetmetal and leaving a new bare surface for rust to begin again. I live in the "rust belt" So I'm all too familiar with rust! This is the best way I have found to deal with rust. I was skeptical about "gluing" panels before doing it this way the first time. But do some research and you will find it to be just as strong as a weld and non- destructive to rust preventatives that are in place behind the panel.




This is on a 1992 Subaru SVX. You can see I had to rebuild the inner wheel house before gluing the outer patch panel on. I welded the patch into the inner wheelhouse but if you get fancy, you could glue that too.




Not a real great picture of the finished product, but it's all I've got.



If you just go the Bondo route, plan on doing it every year! Because it's only going to last about one season before it rears it's ugly head again.