The main problem is that many tanks are not properly installed in the first place. The top surface is flat and salt water which comes in from the engine air inlets sits on top of the tank, and gets on the plywood or other substance that the tank is set upon. There is no way to get the bottom, sides, top or back of these tanks. Much of the rust is from the outside--not the inside. In reality very few fuel tanks have a sump and proper drain. The reality is that many of this boats have to have the fuel tanks replaced--and hopefully the owner will do a better job of puting in a tank: beveling the top, putting the tank up on strips so it breaths and water drains out from underneath and using a corrosion resistant alloy, with appropiate coatings. Bob Austin