It sounds almost hard to believe that water can stain fabric, but that is absolutely possible with satin. Originally derived from silk, satin is now often made from polyester, acetate, nylon, rayon or some combination of these synthetic fabrics. Satin usually has a dull side and shiny side. The shiny side shows a water stain the most and can be the most difficult to clean.
Mix a capful of gentle fabric detergent in 2 quarts of warm water to make a cleaning solution.
Dip a clean, soft cloth in the solution and gently blot at the stain. You are trying to remove minerals in the water that have created the stain. Avoid applying too much solution to the stain since that could make it worse.
Blot the entire area surrounding the stain to get the satin evenly damp. Allow the satin to dry thoroughly before continuing on to Step 4.
Apply 1 tablespoon of white vinegar to a clean cloth. Use this cloth to wipe the section of satin you cleaned. The white vinegar will restore the satin's shine.