Expedient way to stop rust on body panel

Question

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.)

Answer 1

Rust is coming through from the inside; panel is penetrated. Likely mud is collected on the inside. The plastic inner fenders should have stopped the mud, are the inner fenders intact ? An excellent repair is to replace the fender or cut out several inches and weld in a new section ( + prime, sand, paint ,top coat ,etc.) . At the other end of the spectrum is : grind smooth, apply phosphate conversion coat, body plaster , sand, paint and preferably sell the car before the repair falls off in about a year. There are a few intermediate options such as fiberglas/epoxy.