I was reading an article by David Pascoe, the marine surveyer, about how to install an aluminum fuel tank. He has the whole thing on his website, along with many other articles which consistently appear well written and well thought out.
With regad to aluminum tanks, he recommends that strips of frenolic or other plastic type products be bonded to the under side of the fuel tank using 5200 and a completely full layer of it to prevent moisture from becoming entrapped in contact with the aluminum and simply setting them on supports. He said that the weight of the fuel and the physical limits of motion will prevent any moisture from being trapped in contact with the aluminum and some space is good to allow the circulation of air so that any moisture that does contact the aluminum evaporates.
Just passing it on. Check the article out.
http://www.yachtsurvey.com/fueltank.htm