The picture below shows the damaging effects that ethanol has on fuel tanks:
The test was performed by adding a 92 octane (Ethanol Blended Fuel) to a 2002 / 24 ft SeaRay Sundancer fuel tank that had never used an ethanol blended fuel before. After adding the blended fuel, the boat was ran for six hours over a three day period. The fuel line was then pulled from the tank and the the tank was drained.
The picture shows the deposits removed from the previously clean fuel tank. The debris was tested and found to be varnish that had formed a fine layer in the fuel tank. The ethanol had dissolved the protective layer of varnish from the interior of the fuel tank. The flakes had contaminated the entire fuel system including the in-line fuel filter. It took nearly two weeks and ten flushing to completely clean the flakes of dissolved varnish from the aluminum fuel tank.