O.K
More from me!
My old man old school mechanic, [and petrol head] told me, if you want to solder a petrol tank make sure the soldering iron is hot enough to solder BUT will not cause a piece of newspaper to light if held up onto it.
Plus good idea to defume it first. I welded several truck petrol tanks [the loader drivers ran into them a lot], my method of defuming as follows-
Empty all the petrol out DOH!
Take out the gauge tank unit and any removable fittings. The can hide a drop of petrol
Flush out with running water.
Get a piece of radiator hose and connect the exhaust pipe of a running truck to the filler neck, Ive used petrol engined exhaust.
Leave the thing running until the tank is to hot to hold your hand on.
This achieves evaporation of all the petrol left liquid in the seams of the tank and fills the tank with CO.
Now before welding I would stand aside from the filler neck etc, while the tank is hot and poke the flame of the oxy into the neck to ensure no surprise!
My method for soldering Motorcycle tanks, Remove all fittings ,flush out with water,Put your foot on top of the tank, lean back out of the way and stick the flame of oxy down the filler hole. It will flair. When its stops, instantly if you flushed it thoroughly, stick in again, and it mostly won't flair again. Leave it heating the tank till its hot to touch then weld .
Two points 1 I'm still around 2,But get someone else to do it.
cheers pancho