These are the blowholes I got earlier today when I tried to TIG weld end plates into these 50x25mm (2"x1") mild steel sections. Where the sawcut is - that is where I got some blowholes just in that region yesterday - the cut is to mark it so I knew where to come back to and rectify it.
The other end of the section is open. The reason for mentioning this is that if the other end were fully closed, the heat of welding would cause the air trapped in the section to expand, blowing out through the weld region. But this doesn't apply here.
I tried to weld from where the previous blowholes are towards the right-hand corner. The other welds in view were done with fairly much the same preparation conditions. I couldn't get any emery cloth down the smaller dimension internal face of the tube, so filed then wiped with a cellulose-thinners solvent, as I did round the rest of the internal surface.
The other, clean, welds were done immediately before.
Scale - the width from left to right between the two TIG welds is the 25mm dimension, between weld centres.
I solved the problem by slitting away the weld with a cutting disc in an angle grinder, leaving no more blowholed metal, then filled the cut using the TIG torch and filler metal. Came out absolutely clean then.