First, I'll move this topic to the "fuel tank" forum.
Second, from a metallurgy point of view, aluminum develops a pretty corrosion resistant coating when exposed to air. Keep this in mind as you will need to remove it in order to get a good bond between the tank and the product you choose to use.
Lastly, tomorrow I'll dig up some threads with the different coatings other members have used. Myself, I chose to leave the tank natural. I rely on the oxide barrier mentioned earlier and keeping the tank compartment as dry as possible. Check the tank forum for several different styles of installation.
Good luck.
