Pitting in Tig welds? Impurities?

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I'm welding together a role cage for a school project.  Every once and a while the puddle pops a little and then afterwards theres a pin hole or two in the weld.  I'm guessing this is from impurities.  It doesn't seem as bad as when you obviously hit a contaminate but I'm thinking I must be hitting small contaminates?I'm using a 250a scratch start "ArcWeld" machine.  Its a pretty ancient machine but seems to do the job.
Reply:TIG's got to be clean. Make sure your base metal's very clean (lots of folks use acetone). Also there can be a lot of impurities in mill scale on your base metal, best to sand or brush that out close to the weld...
Reply:That is likely the air inside the tube getting hot and expanding as you close off the escape hole the gas pushes out of the puddle?
Reply:Hello Luke, I would agree with both of the previous posters. GTAW has got to be clean, clean, clean, and as FatBastard included, you can have expansion of gases in cases where you are closing an un-vented chamber that can play hell with finishing out a weld. To address the un-vented issue, once you get to the point where you are almost closing up a joint, stop, weld somewhere else or wait until things have cooled down.You can come back to it and finish off the closure without overheating the enclosed area enough to cause the blow-outs. Otherwise, consider drilling a vent hole if your design will allow it.You mentioned a roll cage, if the pipe/tubing that you are using has shellac on it or some other sort of anti-rust agent you could be seeing this melting and running into the backside of your weld joints from the inside of the pipe/tubing and causing the pinholing. Might require some really good cleaning of both the inside and outside areas. Just a few other thoughts for your consideration. Best regards, Allanaevald
Reply:Where one tube meets another drill a small hole in the middle of the receving tube for the air vent and all that "Pitting" will stop..Do this at every point and eventually you will have an escape route for hot air..Also clean up inside each tube as far as you can also..Oil and anything greasy will "Wick" to the heat and come up from behind.....zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:Thanks for all the tips.  I'm guessing the problem is dirty metal.  We have been cleaning the scale off the steel but then we just wipe it down with a rag.  We've also only been cleaning the first two inches or so of the inside of the pipe.  I'm guessing oil has been running down the pipe over time.I'm going to start using acetone to clean up the pipes before welding.  So far none of the welds are complete so pressure shouldn't be an issue.