I’ve used a rust neutralizer that you can buy at Farm and Fleet in the automotive section on my wifes honda prelude, but once it starts to rust I would have to do the fix every spring.
1. mask out the area and lightly sand out the area.
2. clean it up and paint on the rust neutralizer (the area will turn black)
3. Remask and shoot a couple of coats of gray primer
4. remove the mask and wet sand the paint edge of the primer.
5. remask slight further away from the gray paint line.
6. shoot a couple of coats with the body color.
7. remove mask Wet sand the area and the paint edge
8. Mist 1 coat (holding the can about 18″ away) over the paint edge to blend it in.
My rust was behind the rear wheel, started out as pinholes, and when I sold it the hole was the size of a golf ball (fiberglass mesh and bondo fixed that). I saw the car a couple of time this winter and the hole was the size of a football.
My first experience with bondo was when I was 4 years old helping my dad fill about a dozen rust holes in a 1971 Ford Pinto that my dad bought for 300 bucks (drove it for 3 years and sold it for 500 bucks)
Photo of the actual Pinto (notice the rear wheel well looks like it is made entirely of bondo