Here's how I have done it with a solid just inside surface rust, metal solid. just rusty. Wash as much as you can out with hot water, let it drain and dry out some. Get a couple handfulls of nuts, bolts and washers into the tank along with some muratic acid and shake it all around to knock loose what you can. You can clean up a pretty nasty tank this way. After you are done with the acid, give it a good hot water rinse and blow it dry. That gives you a clean bare metal gas tank. I have tried sealers and never had acceptable out comes. It wont need a sealer if you keep gas in it, a new tank is probably bare metal too.