The fuel tank has a life of its own much different from body sheet metal. If you drive the car regularly the interior of the tank is constantly washed with gasoline. The fuel also creates nearly inert fumes in the tank that displace air (no oxygen present). As such, a regularly used car will not have any rust inside the tank. Keeping it well painted on the outside can preserve it for many decades with no other rust treatment required.

My MGA with 390,000 miles still has the original 54 year old fuel tank doing well. In 1977 I washed it, cleaned and repainted the outside with Pettit Polypoxy, a 2-part epoxy enamel, same as I used on all underbody sheet metal and all chassis parts. I also coated the inside with slushing compound, which was probably a waste of time and money. This process has caused problems for a lot of people. With advent of 10% alcohol in the fuel, the interior lining can be gradually dissolved. I think I was fortunate to be driving the car a lot all through the 1990's, running several hundred gallons of fuel through the tank every year. Apparently this dissolved and washed away the lining gradually holding the removed material in suspension in the fuel, and flushed all of the residue through the carburetors along with normal fuel flow. By the late 90's all of the lining material was gone without a trace that it had ever been there. I have never had a clogged fuel screen or clogged filter in 240,000 miles over the past 25 years. Other people were not so lucky. For a car that is not driven much, the lining can come off in large thin gummy sheets and clog up the pickup pipe and/or fuel pump check valves, and/or screens in the fuel pump and carburetors. This may be a greater problem if the car is stored for six months or more.

More recent formulations of slushing compound are claimed to be resistant to alcohol. That may or may not be true, but I have no way to predict what chemicals may be added to motor fuel in the next 20 years (like MTBE for instance), so I would recommend putting nothing inside the fuel tank other than motor fuel.

In 2008 my tank was cleaned on the outside only and painted again with Chassis Saver, which is an asphalt based air-cure (one-part) epoxy paint, same stuff I used on underbody bare spots, and on the frame. The key to long life for the fuel tank is to keep it well painted on the outside, and run some fuel through it on a regular basis to keep it washed clean inside.

Addendum, October 2012:
If you do feel inclined to coat the inside of your fuel tank, someone suggested 'Red-Kote' Fuel Tank Liner from Damon Industries, Inc. I have not used it, but other people report it works well as advertised. You can at least read the manufacturer's information and application sheet.