While tig welding a tank made of 16ga. mild steel I encountered porosity that was only solved by using 304 wire. I ground the mill scale of the mat'l prior, but it didn't help. The welds were outside corner to corner. I have used stainless filler before to correct this problem. Just wondering if this practice is acceptable. Any suggestions appreciated.
1) Is this a new or used tank? 2) If used, is it cleaned out thoroughly(ie. solvents, degreasers etc.) 3) Is this the last weld you are making? If it is you might be getting back pressure from the heated air inside the tank.
bmass1, this is new mat'l. It is a tank for holding liquid in that it is open on the top (bottom with 4 sides). I used a grinder with a sanding disk to remove the millscale. As soon as I would strike the arc the puddle would bubble and spit. Thanks for your time.
It almost sounds like your tank is not deoxidized steel.
Non-killed or "rimmed" steel will often jump right onto a gtaw electrode as soon as melting begins. The deoxidizers in the 304 might just be quieting this effect down.
But thats a big jump.
1st check the simple things like gas supply, wind interfearence etc.
I would also consult an engineer or engineering doccuments to see if your selection of 304 is a good one. More often than not, folks welding mild to or with stainless select 309.
This might be a possibility but it is a stretch. If everything else is ok(gas, cup, wind, etc.) you might be superheating the material on the corner without knowing.
Sraight of the top of my head, I tend to agree with Lawrence on this one. I had the same problem several years ago and it drove the entire welding shop crazy trying to figure it out! Until it was discovered that we were trying to T.I.G. weld 'rimming' steel.
Check all the other parameters mentioned especialy 'gas flow'. Outside corner joints typically need a raised gas flow to counter deflection of the gas away from the weld area. But, if the weld made with the 304 s/s (308 would be a better choice) is nice and clean etc, I would definately start looking at the grade of mild steel your using!
If it is 'rimming' steel, then your options are limited as regards T.I.G. welding it! It just will not weld successfully using 'normal' mild steel filler wire!
You say you are TIG welding. 
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