If it's just light surface rust and no heavy scale a tank full of E10 blended fuel will scour it clean. I'd be tempted to give it a good wash with kero or mineral spirits, and as long as there weren't any chunks of stuff, add some blended fuel and a second filter between the fuel pump and the carb and monitor the flow of crud. I pulled the sender on my 2 year old replacement tank and the inside was shiny.....as was the entire sender unit.
That's one of the few "pluses" I've discovered with the E10 fuel....your tank and lines will be clean. You just need to take extra measures to catch the crud before it gets up to the carb.
Dave
