Hi Glen. For gas backing, I've used little temporary shields for some
welding tasks. The shields can be almost anything, as long as it contains the
shielding gas.
I've used sheet metal formed into little trays, and some times even
used cardboard and duct tape.
This was always used when working with thinner sheet metal like sinks or
duct work on stainless exhaust systems.
I simply installed a "T" in the gas line and ran a 1/4 inch tube to the
shielding device, gave it a few seconds to purge the air out, and set it for a
slow bleed to keep the air out.
These were so cheap and fast to make that I routinely just threw them out
after one use, but over the years did have a small collection of chambers that
I kept around to fit basic shapes like flat objects, 10 inch round, and other
common shapes encountered.
I even used tin foil sometimes.
On high purity round ductwork where it would be xrayed by an inspector, I
used to insert a rubber balloon as a "stop" inside, weld the duct seam
after the space was purged, and then pop the balloon just before welding the
last 1/4 inch or so. If it was VERY critical I would just leave it in place
and blow them out with a fan when I was pressure testing the duct "run".
The balloons were nice because they would conform to mostly any
shape.
I haven't needed this on my sub, as the welds are all "back ground" and a
weld cover pass is applied but for some applications, back shielding helps a
lot.
As for the diatomaceous mud.....haven't tried it, but it should work
fine.
Frank D.