‘Angular’ stains are distinct from the smears or sheets associated with stains caused by water chemistry because they are caused by chemical reactions BEHIND the Fiberglass shell.
- Poor Construction/workmanship. Fiberglass may be too thin in places resulting in oxide leaching from rusting metal reinforcing behind the shell.
- Low quality RESIN used when glass was laminated in construction of the shell
- Age of the shell
Consult a reputable pool renovator.