CM,
[quote]"Now, as far as air being pulled in, that is pretty unlikely unless you have your shielding gas so high that you're creating a vortex effect in the weld zone."[/quote]
I know he mentioned that he was running around 50cfh and most of the GMAW I have been around is more in the 30-35cfh range depending on a couple of things. While 50 is towards the upper end, it may just be too much flow for what they are doing. Joints that run along close to an edge may need that kind of flow for good coverage but then if boxed in a tight corner it may cause a turbulance if ran that high.
He also mentioned fit up and if there is a nice size crack between the plates, he may be getting air sucked in from the backside.
I had problems here when a particular welder would run his voltage in excess of what the FCAW wire mfg recommended and all those porosity problems mysteriously went away when we reeled his parameters back in.
Preheat is extermely important on thicker materials but as long as you follow the preheat guidelines (in D1.1:2004, Table 3.2) he should be OK, unless the materials have an unusual amount of moisture on them.
Hopefully he'll be able to put his finger on it and find his problem. I know those mysterious problems are worrisome to figure out when they come and go.
John Wright