Try buffing them with a scotch brite pad or some fine steel wool. Use some thin oil like 3 in 1 as a lubricant. Years ago a buddy of mine had a brand new motorcycle. His wife was on the back and she used the exhaust pipe for a foot rest. The heat melted the soles of her shoes all over the mufflers. He tried to clean the melted mess off his pipes with a scotch brite pad. Got them clean but he was left with a satin finish. Use the oil to keep from cutting so deep that you start to see the brass under the chrome.