I filled the rust-out in the driver side rear wheel well with GreatStuff expanding foam. This stuff expands incredibly and is very sticky! I wore disposable nitrile gloves — this is important advice.
Here is the finished product after I put Bondo over the expanding foam. I used a series of sandpaper — formed with a 40 grit, then progressed to 180, and finished with 2000. In the forming sanding, I attached a sanding wheel to my electric drill, and wore protective eyegear.
I used spray can primer and spray can topcoat.