[JayJay]

So to get off on a tangent of safe welding on fuel tanks (I know the OP asked about firewalls, not gas tanks, but we've gone adrift a bit) - something I do know a bit about (I spent most of my career as a safety engineer) - there are two different methods of doing this safely. In an effort to help us all learn something, here goes a brief discussion:

Gasoline liquid itself does not pose much flammability hazard - it's the vapor that's the issue, not the liquid (that's why there are jets in carburetors, to create vapor from liquid). Gasoline is a mixture of a bunch of primary solvents like toluene, benzene, ethylbenzene, and more, but in general it takes a mixture of ~1.5% to 10% gasoline vapor in air to be flammable if an ignition source is present. (This can only take place if the vapor/air mixture is above the flash point, which for gasoline is ~-45F. Diesel, on the other hand, has a flash point of around 125F, which is why you use glow plugs to heat it up to start ... but I digress).

So in theory, if you keep the amount of gasoline vapor in the gasoline/air mixture below 1.5% (too lean) or above 10% (too rich), you are golden to introduce a source of ignition - spark, flame, whatever.

In practice, you shoot for 25% of LEL for safety to account for possible vapor density stratification. So this would mean that an atmosphere of ~0.40% gasoline in air would be safe. That's the concept behind inerting the inside of the tank with dry ice. Gas CO2 displaces air. A pound of dry ice (solid) creates around 6 gallons of gas CO2, and you should use at least 3x the volume of inerting gas as you are trying to inert. So a 10 gallon Model A fuel tank would need about 5 pounds of dry ice, completely converted to gas (warmed up). If you go with this approach be sure to seal all openings (fuel gage, fuel valve, fill neck) - I would leave the fuel neck slightly ajar then tape it closed after the dry ice has evaporated. That's the concept CWPasadena notes in #7.

Inerting is not practical on a multi-thousand barrel stationary tank in a refinery. So the welders take advantage of the upper flammable limit and work with the atmosphere too rich - above the 10% UFL. At room temperature the saturation level of gasoline vapor (level of vapor above the liquid, having displaced an equivalent amount of air) is ~40-60%. That's not compatible with breathing, so the welders use a supplied air welding hood that doesn't leak air into the local environment. That's what Kevin in NJ is talking about in #17.

Having said the above, welding on a gas tank is not a good idea unless you are really, really sure you know what you're doing. You don't get many do-overs if you make a mistake.

Now, back to the news...

JayJay