KBS make a tank cleaning, corrosion treatment and sealing kit.
http://www.kbs-coatings.com.au/gold-sta ... structions
I have just finished treating the tank in my '65 Fairlady with the large kit: suits tanks up to 45L.
I got the kit from my local auto paint shop for $59.
Easy enough to use and the tank ends up with a flexible impervious coating which is clever enough to seal pin holes if there are any.
You will need to plug all holes and protect any threads from the sealer.
I pushed some stiff wire into the fuel pickup connection [from the outside] threaded a rubber plug on it, tied the wire ends together and pulled the wire back out to force the plug over the end of the pickup tube.
Once I had set the tank up to drain and cure, I pulled on the wire to remove the plug.
Worked a treat
Follow the instructions carefully!
If your tank is really bad it may require more that one lot of cleaner.
Once it cures, it is there forever