Rust or Not?

If I remove the bubbles and sand down to metal and have pin holes, then there must be rust between the outer fender and the wheel housing. How do I stop the rust without ripping open the complete fender lip.
I'm not a body guy by trade(disclaimer)
You can't really, you either leave it or hide it(like someone mentioned earlier and keep it out of the weather) or you tear into it knowing it could end up needing a whole quarter. If you patch start small and increase the size until you get to clean metal on the inside of what you remove
Be careful this is where the snow ball starts to pick up speed