|I've learned the best seam finish for lingerie, sweater knits, and the like is the stretch wrap stitch on the serger. The high-end Elna's have this built in as a stitch option. I can duplicate it on my very basic New Home 434D by setting up for a narrow 3 thread stitch. The needle tension is set to 3. The upper looper (using woolly nylon thread) is set to 5, and the lower looper is set to 8 with regular cone serger thread.
This makes the woolly nylon wrap around to the back of the seam and the lower looper thread disappears. The results is a very narrow, flat, and soft (thanks to woolly nylon) stitch. It looks RTW and only uses one spool of woolly nylon to cover both sides of seam. Very Cool!!!
I do like to machine stitch a narrow zigzag (for strength) prior to the seam finishing described here.