How to remove rust from gas tank

How to remove rust from gas tank. My gas tank is so rusty that the fuel in my clear in lin filter is brown and nasty. I have to change filters once a month. I can drop the tank but don't know the best product to clean it and prevent the rust from coming back. The car was sitting 7 years before getting a new 305 engine put in. I would like to do this as cheaper than a new tank which cost 100.00