How can i stop cast iron tea pot rusting?

i have a japanese cast iron tea pot which has been excellent to use, but my son has put it through the dishwasher and it came out rusty.I am assuming it must have had some sort of tasteless coating on the inside before because it just tastes rusty now.I love this tea pot and its a great reminder of a great holiday can I re coat it in something or is it ruined? please help !!!!