Call around to your local radiator shops & see if they can weld aluminum. (I'm assuming your tank is aluminum) Most radiator shops are able to pressurize & submerge a tank to locate a leak by the trail of bubbles then weld it up.
You could also coat the interior of the tank with "Kreem". Be aware that modern gas can break down some of the Kreem type of linings, so make sure that you find one formulated for todays fuels. Kreem can usually be found at most Bike shops or online. Eastwood also offers a tank sealer type product.
http://www.motorcycle-superstore.com/1/ ... Liner.aspxPlease let us know what you find out. I have a tank that leaks also. I just haven't had the time to pursue the remedy yet.
Good luck
~JM~