Finally got around to doing a more scientific test. I took a bad part and cut the welds in half on both sides. Then sanded it flat, from 60 to about 400 grit. Then put it in a small glass jar of hydrochloric acid (muriatic acid, $6/gallon pool store) for about 10 minutes. removed with pliers, hosed off with windex, rinsed in cool water, blow dry. Of course I used gloves, long sleeve shirt, and full face shield.
It revealed the depth of weld penetration really well. It appears that the center is the worst part of the weld. I ended up with acceptable (to me) depth at start and end. The start is where I first held the weld a split second longer and the end is where I backtracked the weld a bit.
I sliced out the center area on the other side (least penetration) and did a pull test, bend test, twist test. I only ended up tearing out base metal, so I think it will pass. If I run another lot, I'm going to figure out the adjustments for amp and wire speed (on the fly adjustments) so I can fine tune the welds on this thinner stuff better.