Rust spot

All,
I am in the process of prepping a car to paint, it is a low budget old VW bug. It has many spots coming through the current paint. Have taken a DA and smoothed them down to metal but the rust mark never seems to go completely away. Though the project is low budget I still want the paint to last as long as it can. How should I prep these spots to keep them from coming back. I posted a couple pics showing a rust spot before DA, and another after using DA. What should I do to prep for paint?
Thanks,
Andy

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