A guy I knew who repaired fuel tanks for a living would drain them and then run a hose from a running car in them. The carbon monoxide would stop any ignition.
Just a thought. No warranties on this.

with my tank, i drained the fuel, dried it, washed it with water a few times, then filled it up with water about 10mm from top where i was welding, and welded the top of the tank, it was empty for about 2 weeks before i welded it.

the tank is dry now... no one asked you to filled it up with petrol and weld it...

Just make sure you pressure test it..... Nothing gives you a headache like leaking fuel

fuel vapor is still stored in the metyal and when is heated let out causing explosion/fire. a guy was killed a month ago when welding tanks which held fuel a few years before. gas escaped inside and BANG he dead.

put an oxy torch on a pole and go over it first from about 10 metres away in an open area

There are places that do weld them up though. I've had a new bottom put on one before. I think they have fluid continuously flowing through it while they weld on it and it still shoots sparks and flames...

I've welded several tanks after a big cleanout with soapy water on a hot day. In all cases they'd not had fuel in them for more than 12 months.

I would not have recommended cutting it completely in two. I hope you are good at tig..even the most skilled mig users will have trouble stitching it back together. IMHO

Mmmmm, thanks for your concerns, but not even the sparks and heat from the grinder could get it to light... I lit a few matches and threw them in there before cutting it open, just to make sure. It had Methanol in it from the previous owner, and still had some in itwhen I took it out. I then let it stand in the sun for a good 2 weeks while doing other stuff, so I'm sure there wouldn't be any vapors left.

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I was hoping someone wouldn't say that... What I did notice though, is that the tank tappers towards the bottom, and when I fed most of the sides into the top part. I could, with a bit of squeezing, get the bottom half to completely enter the top half, so that it creates a bit of an overlap, which should be a breeze to tig/mig. I'm probably loosing a liter or to of fuel volume, but then again I'm not going to do the Groot Trek in this car, only to the racetrack and back.

My question is though, will it rust again? It seems that it was only the Methanol that attacked the metal, and the car did stand outside in the rain for many months. So I'm hoping that it wouldn't rust again. Is there anything one can treat the metal with other than galvanizing, that wouldn't come of?

This I think is quite something to worry about for all of us.
Is this rust not something that will be in all our Datsuns due to age?

Mine was never left dry or driven until completely empty. When it starts to touch the empty line I go straight to the garage to fill about halfway.(Still got to fix that problem of fuel vapor smells in the car when filled pas half mark and driving in left hand corners).

It is good to clean out the tank while you are almost rebuilding the car.
Find out from a welding business if they will be able to do it for you and also a preasure test. Rather pay someone than doing it yourself and sitting with a head ache later on.

Good luck.

Will it rust again? Yes. Thats why tanks are zinc coated originally.
That POR (15)? company sells a 2pack fuel tank sealer kit. Mix it up, pour it in. Turn the tank over, around, up and down. Then pour the rest back out.
It will solve your welding missed pin holes problem and future rust problem.
POR15

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Damnit, I was hoping nobody would say that either... I guess fuel wouldn't make it rust, but that there's probably always a little water in there, which would make it leak**. I'm not planning on using methanol again though... If I do it will have it's own system...

Back to the drawing board then. Maybe I'll just follow through with my original idea, build a custom tank...

EDIT:** rust...

When welding on tanks, insert some dry ice inside. this consumes the oxygen in the tank. which we all know is need for fire and combustion.

think about it

Mike

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Mmm I'm thinking about it, and while your idea might work in a sense, it's flawed. Dry ice is just plain carbon dioxide, CO2, which is cooled down so much that it becomes a solid, it's frozen. Last time I checked CO2 doesn't "consume" oxygen. It might displace it, but it won't "consume" it.