Cerium Oxide is what you'll need along with a multi speed buffer and a lot of elbow grease.
You'll need to do it while the tank is empty, but it can be done. I've done it before. I have half a container of cerium oxide that I'd be willing to sell to you.
Now is the time to take care of it before you have any livestock in it.
Let me know if you're interested in the cerium oxide.