Our Fiberglass / Acrylic Shower enclosure is about 7 years old. Ever since it was new, after a shower or bath, within several hours Yellow/Orange stains develope in it wherever standing water drops are before they dry. Sometimes these stains come off easy with a wash cloth, but others are more difficult. What is causing them.. Hard Water? I don't know but like to know what is causing them and what they are and the best way to remove them (the stubborn ones) without damaging the finish...