My 2007 Mazda 6 appears to be rusting at two points, one in each rear wheel well:
(Rust in exactly these spots seems to be a common problem with this generation of Mazda, but dealers say it isn't covered by any warranty or recall.)
I'm not skilled enough to do good body/paint repairs, but I'd like to do something minimal to prevent this from getting worse (without making the car look much worse).
For example, I can dab it with PermaTex rust treatment, but I don't think that will penetrate. I have a Dremel and a 5" angle grinder. (If what you see here poses a risk of spreading substantially I guess I could do a hack job and probably find some aftermarket plastic trim I can use to line the wheel arch and cover whatever mess I make.)