Novus makes a good compound. not directly for tanks, but works just the same. i actually bought mine from a Harley dealership. there are three levels, 3 is in a green bottle and for deep scratches, 2 is for light scratches, and 1 is just a cleaner. you can get all three bottles on line from their direct site. http://www.novuspolish.com/
it actually works very well. and no need for sanding the scratches down either wich is a very big plus. i just went to the local hardware store a bought one of those buffer things you can put on an electric drill. it takes some time to do, but imo
its worth it.