I also use small whipstitches (probably about 12 per inch) with cotton thread 50 wt. (which I use for both piecing and FMQ as well). I TRY to snag just 1 or 2 threads in each hex as I whipstitch them together - more than that and the stitches will show quite a bit. Matching thread to color that dominates each hex also helps ALOT. I've read where some folks use silk thread of a finer weight for handwork like this which virtually disappears but I am practical, using what I have lots of which is cotton.