Sewing Tip: Finish the individual seam allowance edges prior to topstitching the seam as this will be very difficult to do once the topstitching is applied. Your chosen finish should be applied after the seam is stitched but before ironing the seam allowance layers flat in preparation for topstitching,
The simplest and perhaps most effective way to finish seam allowance edge prior to topstitching them is by using a serging stitch (if you have an overlock sewing machine) or a zig zag stitch available on your home sewing machine. Not only are these finishes perfect for locking fraying fabric edges, they also do not add unnecessary bulk and thickness to the seam, thus providing a smooth, flat topstitching application.
Once the seam allowance layers have been clean-finished, they can be ironed flat prior to machine topstitching. In this instance, the seam will be topstitched on both sides thus both seam allowance layers are serged separately and ironed open as shown.