The proper method for removing a stain will vary depending on the staining agent, but whatever method you use will be more efficient if you pretreat stains by first applying a paste made of 1/3 cup of warm water and 6 teaspoons of baking soda. Allow the paste to fully dry before moving on to washing the swimsuit.
Most stains come out rather easily, but sunscreen lotions can be one of the worst. Here, you may need to turn to an oxygen-based bleach, mixed into cool water. Let the stained suit soak in water for at least eight hours, and repeat if the stain is not eradicated. Remember, this method should be used only with an oxygen-based bleach product, such as OxiClean. Never use ordinary chlorine bleach on a swimsuit.