Cobalt staining is particular to fibreglass surfaces. It usually looks like smooth black spots that have spider veins. This occurs in the fibreglass structure and cannot actually be removed permanently. Usually it is associated with fibreglass blistering arising from the protective gel coat being damaged or pitted.
Consult with the manufacturer of the pool shell for suggestions on how to address the problem. The stains can be temporarily faded with an application of ascorbic acid, oxalic acid and citric acid. Alternatively, maintaining correct water balance and using a preventative maintenance dose of Pool Magnet may help to minimise visible staining.
