I tried the Por 15 sealant in the aluminum tank on my little modified 28 rpu street roadster and it started to peal after about a year and then what a pain in the a$$. I did the etch and all of the suggested prep before I used it. I was cleaning it out of my fuel filter for a while and finally just cut the top off of the tank and power washed it out, then put a huge screen filter before the fuel pump and cleaned it several times and I am finally running. What a pain!!!!
The trick to aluminum gas tanks is to plan the fabrication such that all of the weld joints that are on the lower sections of the tank can be back welded, i.e. making a "wash pass" along the weld seam from the inside, usually does not require filler rod. If your weld bead has a small shrink hole were you stopped welding it will be a leak. Once you are done fill it with water and mark the leaks, empty, fix the leaks and do it again until you don't have any leaks.
Rex