Well I decided to make some subframe connectors tonight.
Some of the welds came out nice, others.... (I'll be really technical here) - others seemed to be full of air and "deflated" as I went, leaving a bunch of little holes in the bead. (the first two pictures show what I mean with the little holes). Since my technique has been under scrutiny lately, I'm offering up pictures of my work in hopes that I'll get some constructive criticism on what to adjust to make the welds better.
I adjusted the wire speed and voltage to no avail. Its a Hobart 220v MIG with 75/25 mix gas, and its incredibly simple to use, just seems to pick and choose when it wants to make nice beads.
The pinholes are from dirt/oil. It's called a gas inclusion,..It's exactly like a fish eye in paint,..and for the exact same reason. The metal you were welding may have had some coating on it as well that caused the reaction.