BE VERY CAREFUL about welding a gas tank. Many people have been seriously injured welding a gas tank because the fumes remain in and around the tank for a long long LONG time (weeks, months, longer) and ignite very easily with the high temperatures of welding. Many shops will absolutly refuse to do it anyway, but just FYI, dont let anyone do it unless you trust they know what they are doing.

Por 15 sells a gas tank coating that they claim is very effective at sealing the inside of gas tanks.

I have had success fixing small leaks on my gas lines (in need of replacement) with an clay-like epoxy (cant remember exact name) that I believe I first heard about through JB Weld. I think the JB version is called JB Patch or something. I didnt find it in JB brand but I found another brand in PepBoys and Lowes. It is basically a cylindrically shapped portion of firm epoxy that you kneed together for a few minutes and apply. It sealed my leaks right away and i havent seen a problem since -- but I still plan on replacing the lines at some point.

Good luck.

-IJF