Here's an answer from a weldor (this is the correct spelling) and mechanical
engineer (with a concentration in welding/metallurgy) of many years.
Stainless steel distorts a LOT from welding. Here's why:
Shortly after it was discovered (by accident, after somebody melted some
chromium and mild steel together . . . the resulting alloy didn't rust very
much at all), it was learned that this material resisted heat pretty well.
In other words, it retained some impressively-high strength even while hot.
So the name "heat-resisting steel" was applied and generally used. But
in common applications such as cooking utensils and liquid containers, where
strength is not as important as chemical inertness, the word "stainless" became
a major marketing tool.
Back to the concept of "heat-resisting" . . . If you can imagine red-hot steel
having about two percent of the strength of steel at room temperature, then you
know that heating the steel makes it quite easy to bend. I think most or all of
us have done this. Stainless steel, by contrast, maintains much more of its
strength even while red-hot. This is why many industrial components are made
of it, for use in hot environments. Stainless steel at 1200 degrees F may have
somewhere around 20 percent of its room-temperature strength.
Now, consider a weld cooling and shrinking as it cools. A weld made of stainless
steel, as it cools (while still red-hot) will pull harder than a red-hot weld made
of mild (regular, common) steel. For this reason, stainless steel has a reputation
for "pulling like a m-f" when the weld cools. In some instances, one can compensate
for this by fixturing geometrically "biased" so that after the weldment is taken out
of the fixture, "springback" can pull the weldment into the desired configuration. An
example might be as follows: If you want to weld two flat plates together to form
an "L" shaped weldment, maybe they should be clamped so the angle between them
is about 92 degrees (obtuse) on the side where a fillet weld will be. When the weldment
cools, it will pull about 2 degrees, closing that 92-degree angle to about 90 degrees.
As mentioned in earlier posts, lots and lots of small tack welds will help. Welding both
sides of the material will also help, alternating from one side to the other (even in tack
welding). But this may not always be practical, as in the case of a small fuel tank. The
tank could be designed so that welding can be done on the inside and the outside of major
subweldments, before the final welding is done (on the outside only of the last seam) to
join the subweldments together, resulting in the final weldment.
For a fuel tank, here are my suggestions . . . 1. Use fairly thick stainless steel, since in
land speed racing, you don't need to be too concerned about weight -- at least compared
to aerodynamic drag and other friction. Also, use stainless filler material and add it while
welding (instead of simply melting the sheets together).* 2. If the walls will be flat and
distortion would be a big issue (usually for esthetics only, in my opinion), you could add
stiffener ribs on the inside or outside of the walls before the walls are welded together.*
I also agree strongly that "back-purging" with argon is a wonderful idea, instead of using
any kind of flux. Don't try to save money by using any other gas for this purging, as it
works best with argon. I can explain later why. There is a lot of experience available
from others. Please consult them. Even purging with argon can be done wrong, so
ask for details about argon flow rate, and "scavenging" during initial purging. And I also
strongly suggest using a gas lens on your GTAW (formerly called "TIG") welding torch.
*You could ask me for more (e-mailed) details if you want, on all of this.