This shirt encountered an accident. I contacted the maker, VOORMI, and sought advice. They were super supportive and recommended me to use a whip stitch, fine needle, and very fine thread. About in the 80 to 100 weight thread, actually, in polyester.
I had some 80 weight Aurifil cotton in the house but I don't think mixing cotton and the wool will be a super good plan. I do know that the spiffy fine cotton thread, with the right (wrong) encouragement, will break. So I ordered new thread spools from Red Rock Threads and Ripstop By The Roll.
I'm about to set up my embroidery hoop for floating the shirt on top of a light water soluble stabilizer. Probably the heavier one of my 2 Sulky wash away ones. (The lighter one, Solky Solvy, probably can't handle being the support layer on the bottom.)
I'm thinking the DMC size 28 tapestry needle will be the way to go. 2nd choice, the Bohin size 26, a tiny bit thicker, and with that amazing Bohin polished surface. Third, a new packet of needles I found at Walmart, branded Singer a mix of sizes 24 and 26, also tapestry tips. The tapestry tip needle is the hand sewing version of the "stretch" needles sold for machine sewing. It will work it's way between the strands instead of piercing them. And gently does it is the theme for this extra fine Merino wool shirt.
I will use tacking stitches to hold the knit steady while I work on the tiny, tiny whip stitches. That is why I called this floating, it's the equivalent to floating projects on the embroidery machine.