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You probably have a couple options. You could repaint it with high heat paint. Or give your firebox a lite coat of cooking oil and then fire it up and let it season.
IMHO If you have rust, you need to do a really good sanding to stop the rust. Then paint it with a high temp paint, There are 3 different paints on the market. High Temp paint, Header/exhaust paint, and Wood Stove paint. I learned the hard way that you should use either header paint or wood stove paint they are made for the higher temps that your firebox will give off.
I recommend doing multiple coats and lightly sanding in between coats. Put it out in the sun for a couple days to bake the paint in before firing up and I would let the paint cure for at least 7 days.
If you know someone who can sand blast it, that would be your best bet for prep.