One way to prevent raw hem jeans from fraying is by using a product called Fray Check. This clear liquid substance is applied to the fabric and forms a temporary barrier that helps to seal in the threads. Another method is to carefully sew a zigzag stitch along the edge of the denim. This will reinforce the fibers and help prevent them from coming apart. Finally, you can also apply heat to the raw edge with an iron or hair straightener; this will cause the fibers to fuse together and create a more permanent seal. Whichever method you choose, be sure to test it on a scrap piece of fabric first so that you know how it will react with your particular type of denim.