How do I get rid of this rust in the trunk?

The trunk of this car was covered in POR15. I began removing it and had to do a lot of rust repair to the passenger side of the trunk. The drivers side metal feels solid but I can't seem to get rid of this pitting that was underneath the POR15. The metal is strong so I don't want to go through

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How do I get rid of this rust in the trunk?

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