For most swimsuits, ready-to-wear or handmade, the edges of the swimsuit are stabilized and finished with elastic. The elastic is sewn to the inside of the suit and then folded over and topstitched. It’s really quite simple to do and with a few extra tips and tricks, you can have a perfectly awesome elastic insertion on your own swimsuit!
When you sew the elastic to the inside of your swimsuit, you should use a wide zig-zag. If you have a stretch zig-zag stitch on your machine, this is a great place to use it. If not, use your normal zig-zag on its widest setting. This will securely attach the elastic to the suit and allow it to stretch. When you fold the elastic over, you can use the same wide zig-zag, a narrower zig-zag, or a straight stretch stitch (also called a lightning stitch) depending upon how you want it to look from the outside.
When you sew the elastic to the inside of your swimsuit, don’t align the elastic to the very edge of the fabric. Instead, sew it 1/16th (or just a tiny bit) in from the edge of the fabric. This way, when you fold it over to the inside, that little bit of extra fabric acts as a turn-of-the cloth to cover the edge of the elastic on the inside so it is invisible. However, don’t worry if your alignment isn’t perfect! The swimsuit on the right is an example of the ideal. For the swimsuit on the left, the lining and main fabric got off by a bit. It’s perfectly okay. The inside won’t be quite as beautiful, but it will be just as strong and functional.
For most of your swimsuit, you don’t want to stretch either the fabric or the elastic as you sew it. The easiest way to make this happen is to keep a bit of tension on the elastic as you sew. I know this sounds a bit counterintuitive, but you shouldn’t be stretching the elastic, just holding it taught as you sew to keep the edge of the suit from rippling after you install the elastic. If you’re a knitter, think of it similar to how you hold your yarn when knitting – you’re not holding it tight but instead just keeping a bit of tension on it as you go. The difference is subtle, but you can see that the bottom sample lies totally flat while the top ripples a bit. When it’s on your body, the difference won’t be noticeable as it stretches a lot to fit your body regardless.
The one place that you do always want to put a bit of stretch in your elastic while you sew is around any convex curve. This is often the top of the swimsuit cup and around the cheek on the bottom (although some patterns, like the Nautilus Swimsuit, give you specific instructions about stretching the elastic more in certain areas. Follow the instructions first!). Putting a tiny bit of stretch in the elastic as you sew allows the fabric to grip nicely to your body instead of gaping away. The difference is subtle if you are looking at the fabric flat, but it really makes a difference when you place it around a curvy part of the body.
The best way to figure out the difference between putting tension on and stretching your elastic is to practice sewing on some fabric scraps. The settings and feeling that you use will be different depending upon your fabric, elastic, stitch, and sewing machine feed. If you find it helpful, Erin from Tuesday Stitches has put together a little video of what it looks like.