I have had luck with a green scotchbrite pad on an old cast iron frying pan. Scrubbed all the rust out, then re-seasoned it in an oven. If rust is thicker I would suggest sticking the dutch oven in a campfire's coals for 20 minutes or so, then scrubbing after it cools with scotchbrite pad, or a wirebrush wheel in your drill. If the cooking surface is badly pitted, then all bets are off though.