dissolve some "Iron Out" in a sink of warm water, and then immerse a cotton terry towel in the sink. Bring the wet towel out to the car, and allow it to lay on the stain for 15 or 20 minutes, and then lift it up to see if it has absorbed the rust from the paint. It might take multiple applications to lift the rust off the paint. You can buy Iron Out at Home Depot or Lowes. I used it to remove rust stains from a pillow case that spent years in an old ambulance linen cabinet. The car was so badly rusted, that there were holes in the roof.