I just went after it, and I discovered even at higher powers TAC TAC still works with difficult welds.
The uprights on the gooseneck trailer are not vertical on the front. I'm taking two piece of 1/4" (6.35mm)thick angle to make the equivalent of a piece of channel across the front that is vertical so I have a vertical surface to weld the jacks to.
I tacked the top of each piece, measured to the top edge and bottom edge of the trailer frame to get them perpendicular to the frame, adjusted with a hammer, and then tacked the bottom.
The tacks went fine. The horizontal weld on the top went fine. Even given I had to fill a gap left by a fishplate/repad. Vertical up went terrible. Vertical down went marginally better, but you are suppose to weld FCAW with a trailing angle due to flux inclusion. Neither made me happy. I welded the two pieces together in the front by stacking tacks as if I was welding pickup beds or sheet metal. Except for being a bit taller than I might like it looked pretty good. It also cleaned up decent. Well for a shade tree farm mechanic anyway. Of course I had to then grind that weld flat which showed my terrible results. Not as bad as you might think. In 12 inches of weld I did have a few tiny pockets exposed, but only a few. 4 or 5 at most. Probably flux/slag inclusions.
I've done vertical welds with FCAW before, and I have welded over head with it. The problem is I don't do it often enough to remember what worked last time, much less develop any muscle memory.
Now to make the mounting surface for the other jack... after I have another glass of water. Its hot out there.
I am not looking forward to welding the jacks on. That will be another
48 inches of weld... probably stacking tacks.