Good idea, he could also use wet paper.

This has nothing to do with welding a gas tank.

I'd suggest using a torch with a pointed flame, should be able to get the soldering done before everything gets too hot. Solder has different melting points depending on the composition, but it's around 400 degrees F. Red heat is quite a bit hotter than that.

Clean the area to be soldered and use the correct flux and it'll be OK.

R.