i have an 84 gn, and i want to know how to check and clean the stock fuel tank for rust and water, and how to prevent this from happening.
You can pull it out and have it cleaned and coated by a radiator shop. If it is rusting thru, then I would buy a new tank.
Once the tank is back in the car, keeping it fuller will help but you can also buy "Gas Dry" at the the parts house and add that to the tank..it is basically methanol and absorbs water from the gasoline.
Simply unhook the fuel lines going to the tank and the electrical connector. Then you loosen and remove the straps as the tank comes down. Remove the soft "cushion" straps from the tank and visually inspect for rust. If there is some minor exterior rust, there is a procedure for cleaning and repairing the rust on the gnttype web site that you can use to repair the tank. As steve mentioned, if it is badly rusting...its junk.