All right, I finally have the etches...
From top to bottom, Stick (6013), TIG (70s-2), and MIG (70s-6). The coupon was A-36, 37* bevel to knife-edge, 1/4" A-36 backer, 3/16" gap.
My immediate impressions are:
The 6013 showed it's low penetration, as the root (3/32 @ 90A DCEP) has very low forward force into the backer, but cuts fairly broadly into the sides. There's also a slag inclusion obvious. I may re-cut it later to see if I can find a clean surface. This was seven passes, with the last one intentionally wide of the opening to see the penetration on pre-heated metal.
The TIG was the most consistent, with good penetration into the backer, and each of seven passes are clear with their relative penetrations. This was 1/16 70s-2, 3/32 thoriated, 20cfm, #8 on a gas lens, at about 105A all the way out.
The MIG also has good penetration, but betrays my biasing of the arc to one side of the root (perhaps not fully perpendicular to the joint), as shown by the step on the right side at the backer bar. There are also two tiny silicon inclusions, also. These things show me me that, even for this exercise, I should have paid more attention and cleaned better between passes. This was .035 70s-6, 20.8V/350ipm, 70/25 at 35cfh, about 1/2" stick-out.
This is the first time I've done a cut and etch on my carbon work, as I work almost exclusively in stainless, and I've frankly learned a lot. I may have to repeat this exercise. If I do, the stick will be done 6010/7018 (I can hit up the LWS for some samples), for contrast. I may even do spray-arc MIG, to contrast that.
Any and all comments are welcome.