There has been a recent thread running on Origami Boats discussing the fire retardency of duct tape... huh? Well, it seems that somebody did a quick patch on a hole in their pants with duct tape...worked fine until they were welding and a hot spark dropped there... seems duct tape is rather flammable. They batted at it with their hand and got a ball of hot metal in their palm for their efforts. In the end, no serious injury, but a fair warning.

In the on-going thread there has been discussion of other fireproofing methods. One suggestion was starching the clothing to improve its ability to resist sparks and flying metal globs... might be worth a shot.

Most recently a product called PROBAN was recommended by Colin (below). I haven't found a source of the chemical itself yet, but many clothing suppliers sell products pre-treated with it.

I bought some not-too-expensive overalls a while back, treated with some stuff called PROBAN. I was a little sceptical at first of how good this stuff was, so I tested 'em with a shower of sparks from a 9" grinder. (with a bucket of water at the ready) Heat under the material very quickly became unbearable, but no signs of burning or deterioration on the surface.

Guys in the race-car business claim that PROBAN-treated coveralls are good for around 75 washes, after which you're left with cotton. Their preference is for Indura/Nomex suits, which are good for walking into fires with. But for welding and general fabrication, PROBAN appears good enough.