We have what we assume are metal stains since Vitamin c will remove small areas. They are covering the walls of our above ground pool and are in various spots of the floor. We've tried MSR and Stain Free with heavy brushing and nothing seems to remove them. Now, we are noticing white spots on the floor that look like the liner is getting bleached out. Have we only just begun and are there lots of methods to remove these stains? It looks terrible. 
