Mini Tutorial: How to sew a hairline seam finish
A hairline seam finish is completed with a zigzag stitch and is used with lightweight, sheer fabrics to eliminate seam allowance bulk and simultaneously prevent the seam raw edges from fraying. A hairline application is most commonly used to clean-finish sheer self-enclosed edges as opposed to functional garment seams.
1. Start by aligning and stitching the seam edges in a conventional fashion, at the designated seam allowance width. In this instance, the seam is stitched at 1/2" seam allowance. Before proceeding to applying the zigzag stitch, hand baste the seam allowance layers together along the center as shown above. While you can skip this hand basting stitch, if working with flimsy fabrics, the addition of this temporary stabilizing stitch will keep all layers stable for a smooth zigzag application.
2. Apply a zigzag stitch right next to the seam straight stitch as shown above. Use a higher stitch density and a longer stitch if working with fabrics that fray more when cut. It is always a good idea to pre-test a scrap of your fabrics in order to determine the most appropriate zigzag stitch setting.
3. To complete the hairline finish, trim the seam allowance layers next to the zigzag stitch (at about 1/8" away) to eliminate most of the seam allowance bulk.
The end result will be a narrow, slim finish that aside from eliminating most of the seam's excess bulk is also treated for fraying.
4. To complete the hairline seam: If the application is use don a regular seam, iron the seamline with the narrow seam allowance pointing to a single direction as shown above. It is easiest to iron from the seam's face side.
If the hairline finish is applied on an enclosed fabric edge, iron the finished seam edge with the hairline seam allowance sandwiched in between.