Success! After spending hours eliminating all the possible variables, I've finally found the problem. Ok, actually three separate problems which all lead to the same conclusion: sparks and porosity. It wasn't the base metal or the cleaning or a gas problem. It's a grounding issue. Erm,... well ok it...

I've actually read a whole lot about weld porosity, so my last attempt (yesterday) was a bit psychotically cleaned. :) Firstly ground of with an angle grinder, than filed with a fine file as to remove any sharp gouges left by the grinder, than 99.9% acetone. The inside of the pipe was firstly filed ...

I am now armed with a whole range of different gas lens options. Fingers crossed. The welding gurus at the local welding machine repair shop stake their reputations on there being some sort of impurities in the base metal of one of the parts; in which case the gas lens should help exactly nothing. :...

I've also come across this when there has been slag from a stick weld at the back of a joint, so when you get full penetration with the tig, it draws in some of the slag and sparks start flying. You bring up an interesting point. Aren't pipes initially curved and than machine line-welded together? ...

Thanks Mick. Great idea on the length of the tungsten sticking out. That's one of the things I have not tried yet. I normally have it out about 8 - 12mm. One more observation I've made is that if a put the torch at a rather steep angle, lets say above 30 degrees to the weld plane (i.e. decreasing th...

It's driving me nuts, cause I love welding and am getting pretty good at it especially on aluminium and stainless. It's the mild steel that's driving me insane. I'd be quite happy if it were a problem all the time, cause I'd be able to eliminate the variables to pinpoint the problem one by one; but ...