So this is what I had done for the fuel tank repair on the celica, as we talked about this past weekend.
The tank had rusted through along the rear seam & was leaking fuel anywhere over 1/2 tank. To repair it, I pulled the tank and wire-brushed the area around the leaking seam off down to bare metal, then covered that in JB Weld to create some sort of structure & strength there, and then coated that in a rust-inhibiting primer and then underbody coating. For the inside of the tank, I went with You are not allowed to view links.
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Login. Its basically a 2-part epoxy-type product, comes with good directions for how to prep, clean & coat the tank, and makes a nice, thick, durable seal. The sealer requires a pretty thorough cleaning (which you'll want to do on the inside of a 20+ year old tank anyway), and recommended sloshing acetone or lacquer thinner around inside the tank after plugging/taping over the various holes for sending units, lines etc. I did this a couple times, and then once more with some mildly soapy water, then clean water. Then I stuck a hair dryer in each of the holes big enough to help dry everything out.
Since the sealer is basically an epoxy, you need to be careful how you block off holes and lines. With access plates like the fuel pump/evap & return lines, just taping over the hole works since you can cut away anything that hardens over the hole, but if there are small ports that would be hard to open back up (with a drill, knife etc), then threading something through that you can blow air through to keep them clear is useful.
Then, just mix up the epoxy, pour it in, seal off the hole that you poured through, and rotate the tank around to be sure you're coating all the surfaces.
It's been about 3 years now, and the repair is holding fine. I'd also try to replace or make new gaskets for any connections to the tank, and I had re-made the fuel and rear brake lines at the same time, since they were looking a bit rusty.
