Allow the sand on your white bathing suit to dry completely. Brush off as much sand as possible.
Cover the remaining sand stains with prewash stain remover. Allow the stain remover to remain on the fabric for 10 minutes to loosen the sand stains. Rinse your bathing suit with warm water to remove the prewash stain remover.
Pour 1 cup of white vinegar into a bucket. Add 1 1/2 quarts of warm water to the bucket.
Place your white bathing suit in the warmed vinegar solution. Soak your suit overnight to lift the stains and whiten the fabric.
Remove your white bathing suit from the bucket and set it aside. Empty the bucket and rinse it out.
Pour 1 tablespoon of heavy-duty laundry detergent into the bucket. Fill the bucket with warm water. Dunk your bathing suit in and out of the water to agitate it. Gently scrub the areas that were stained with sand to remove any remaining stains.
Rinse your white bathing suit with cool water until there's no more detergent residue. Lay the suit flat on a clean towel. Roll the towel up with the bathing suit inside. Press on the towel to absorb excess moisture from the bathing suit.
Hang your bathing suit to dry over a drying rack outside in a sunny location or near an open window. Allow the sun to dry and further bleach your white bathing suit.