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Drink the case of Natty Light and go at it with 400 grit. I'm no pro at painting but if you plan on spraying it with primer, you don't need to remove the paint, just scuff it up and smooth it out.
you're not going to get baked on factory paint off using any hand rubbing unless Superman is your cousin. I'd scuff up the remaining paint using 600, then 1000 (all wetsanded with water and a little car soap), then tape off the painted areas around the bare metal and try to build them up slowly with primer to the same level as the remaining paint (sanding any dust that gets stuck to the primer as necessary). Then prime the whole thing a few more times (sanding btw coats), wetsand with 1500 and 3000, then paint. After about 30 days, compound and polish...