Things You'll Need
Stretchy, acrylic blend activewear fabric in a nude or dark color
Fabric marker or chalk
Modern bathing suits come in a wide variety of styles and fits, from tiny bikinis to flattering one-piece suits. An acrylic lining helps you to retain modesty when the main body of the suit is wet or in direct sunlight. Whether you made your own bathing suit from scratch or need to add a little structure to that bargain suit you found at the store, a few measurements and some sewing skills are all you need to add an effective lining to any bathing suit.
Turn the swimsuit inside out and lay on a flat surface. Measure the length of the crotch piece from about an inch above where the swimsuit widens out to sit on the hips on each side. Measure the width of the center of the crotch. Add an inch to this measurement.
Measure the chest area of the swimsuit. Measure across the front neckline, from side seam to side seam. Add an inch to this measurement. Measure across the area on the swimsuit that would hit just below the bust line and where you would like the bottom of the liner to sit on your body. Measure from side seam to side seam across the front of the swimsuit along the nipple line of the suit. Measure from the neckline of the suit down to the area on the suit you want the liner to hit. Add an inch to each of these measurements. String bikini tops can be traced directly on top of the liner fabric.
Lay the liner fabric out on a flat, even surface. Mark out the crotch liner piece using the crotch measurements. Draw an hourglass shape, with the center, slim part of the hourglass matching the width measurement of the crotch. From the top to the bottom of the hourglass shape should match the crotch length measurement. Each wide edge of the hourglass shape should measure about 2 inches wider than the slimmest part of the crotch lining.
Mark out the bust piece onto the liner fabric. Draw a shape that mimics the neckline along the top of the liner piece and mirrors the nipple, ribcage and height measurements from the previous step. The top piece should be designed to fit snugly over the bust area only. Place a string bikini top over the liner fabric. Mark around the shape of each bathing suit cup about an inch away from the bathing suit itself. You need two matching cup liner pieces.
Cut out both the crotch piece and the bust piece. Pin the crotch piece to the matching crotch area on the swimsuit. Fold under the raw edge of the liner fabric as you pin the crotch piece into place. Ensure that the fabric sits right along the interior of the bathing suit so that it will not be visible when the suit is right side out.
Pin the bust piece into place, folding the raw edge of the fabric underneath itself as you pin the fabric in place. Make sure the liner sits snugly along the neckline of the bathing suit without poking out over the top of the neckline.
Sew along the edge of the crotch piece on each curved side using a sewing machine and a straight stitch. Remove the pins as you sew. Leave the two flat sides of the hourglass-shaped liner free. Activewear fabrics are not prone to fraying. Sew the bust liner into place. Sew along the top neckline using a straight stitch and down along each side seam. Do not sew along the bottom edge of the bust liner. Turn the bathing suit right side out.
Make two of each liner piece and sew a double layer of liner into your swimsuit for extra coverage.