Soldering Fuel Tanks

[Tony Smith]

I have one good straight rust-free fuel tank with the filler neck removed, and one battered fuel tank with a good fuel filler neck. How do I go about soldering the good neck on to the good tank?

All the people I know who are adept at soldering (Plumbers and Electricians) just give me a blank look when I ask about soldering steel to steel, as they're more used to working with copper and brass. Is there a particular type of solder alloy I should be looking for? What should I use for flux? My small Soldering Iron won't be sufficient for this job, would a low Oxy flame be too hot?

It has been quite a while (years) since the tanks have held fuel and there's not the faintest whiff of petrol vapour from either. I've cleaned out the tanks with POR15 metal cleaner, a handful of gravel shaken vigorously, then another rinse out with POR15. Do I need to fill the tanks with water to prevent a Whoompf event, as I feel the water will act as heat sink and prevent a good soldered joint?