There were 3 spots where it had rusted through under the seat because of spots where the seat wore through the paint. I've never patched a tank before so I looked into a lot of different methods. The holes were way too big to rely on a coating from the inside and I didn't have much confidence in any of the exterior epoxy type patch products. Gasoline is pretty nasty and it seems to me like anything non metallic would be a temporary fix.
I thought about welding it but I didn't want to ruin the surrounding paint with heat. I watched a few videos and decide to solder it up. I have only ever soldered wire connections so I was a bit skeptical about how it would stick to old sheet steel. I used flux paste, regular electrical solder, and a small butane torch to get these results. A bit of a learning curve here but I think it turned out really nice. I used a small piece of sheet metal to fill in the majority of the biggest hole and then sealed around it with solder.
It's been sitting full of premix about a week now with no signs of leakage.