Another thing to add to this cool discussion. If you are getting porosity in your weld and everything is clean and the gas is flowing, you may be boiling the weld puddle. If you are welding too slowly you can cause a "boiled" weld puddle.
I typically weld with ER70S-6 filler rod for my mild steel welds. I buy 36" lengths of 1/16" and .045" filler rod. 1/16" is for thicker metals 16 gauge and up to 1/8" thick. I use the .045" for 18 gauge steel. Sometimes when I weld thinner metals I will weld with .035". All ER70S-6. ER70S-6 has more de-oxidizers in it than the ER70S-2.
