Last season I used your Ascorbic Acid solution for the yellow stains in my pool after trying an ascorbic acid pill on a small area. It took out most of the stains but not all and the stains returned. I believe it was caused by my pool service crew leaving an Alyn wrench in the shallow end over the winter as I can see the outline of two of these wrenches on the liner. My pool is 16x75 feet, with a vinyl liner. Do you have any suggestions as to how to get the stains out, including the wrench outlines?