The truck tank i was talking about was for a 89 pickup and i think it came from a supplier out of California that sold body shop supplies etc. I cant find the sales order to give the name. I know about the alchohol etc, and i do not know of any sealer that alchohol does not destroy. Yea i know there is different brands that say they work but have not found one that does. The make some for aircraft and they donot work. MarvMarv, original tanks were tern plated, which was a type of galvanizing, a lead/tin rustproofing. Over the years, the tern coating would gradually go away, and the natural thing happened with the tank being steel, they rusted. This is why the sealers were introduced. Today rust is even more of a problem due to alcohol laced fuel posing an attraction to moisture, and the old sealers are incompatible, so new formulations of sealer have been developed.
Sorry, I haven't heard of any "New" tanks having sealer, and why would there be sealer only on the bottom? Niether of these sounds right to me. Not doubting what you said, but you may have been taken for a ride on that "new" tank.
Gary, I think you're playing with dynamite there. Brazing is not joining the two halves together, you're just sticking them together with glue. You need to make the two halves into one whole by welding. We don't want to hear about a '39 Ford that went up in flames in Missouri... and your house... and you.
Tack weld in spots around the perimiter, then come back and spot in between. Keep that up until you have spots close enough to make a weld between a couple, and on and on.
Hell, if you can't git er done, we'll take up a collection for you. Don't braze it!