What is a hairline seam finish?
A hairline seam finish is applied using a regular machine zigzag stitch, which is positioned right next to the seam stitch for a thin, bulkless finish. The goal of the zigzag application is to contain the fraying fabric edges while also maintain the seam strength given that the seam allowance is trimmed so close to the seam stitch.
When applying a hairline finish, it is important to choose the right zigzag setting on your home sewing machine. Stitch density particularly important as it determines the seam's strength and ability to contain fraying edges. A denser stitch is will provide ,more strength and better thread containment. However, it is important to consider the fabric's structure and how it will respond to the zigzag application. It is common for overedge stitches (like zigzag) that are too dense to cause tension in the seam. The basic rule you should fallow when deciding your stitch setting is to choose the highest zigzag stitch density and length that will not cause tension in the finishes seam/edge- as aways, testing a scrap piece of fabric will help you make a final decision.
A hairline finish is conventionally used with very thin, light fabrics, especially sheer ones like chiffon and organza. Given that the seam allowance excess is trimmed so close seam stitch, a hairline finish may not be the most durable application or regular, functional seams that will incur lot of pull and stretching during wear. For that reason, a hairline finish work best to clean finish enclosed edges that require bulk removal. In the case of sheer fabric enclosed edges, a hairline finish a provides a cleaner more professional finish compared to fraying edges left unfinished and showing through the sheer layers.
A hairline seam finishes should not be used with highly -fraying fabrics. Due to the very narrow application and need to trim the seam allowance layers close to the seam stitch, the zigzag finish will simply not hold up, weakening the seam and casing it to unravel or come undone.