How do you remove a stain on the hot tub shell?

Surface discoloration in spas usually indicates the presence of metals, or, for fiberglass spas, cobalt staining

Use of Stain and Scale Control will remove some fresh stains. If, however, the stains have been present for a long period of time, it may be necessary to drain the spa and perform a light acid wash. If the spa is fiberglass, cobalt staining can be temporarily faded by an application of oxalic or citric acid. Usually, the discoloration will reappear. This has to do with a problem within the fiberglass itself, and there is no way to prevent its return.