A common problem with fiberglass pools is that after aging they may begin to suffer from the “black plague”.
Most fiberglass pools are coated on the inside with a clear layer that is called “gel-coat”. This layer protects the fiberglass material from exposure to water, pool chemicals, UV light, etc. Depending on the thickness and quality of this material, it may begin to wear down after time. As tiny, invisible “pinholes” develop in thinner, older gel-coat, water and pool chemicals can react with cobalt in the fiberglass mixture to form a cobalt crystal. The appearance of these crystals has nothing to do with the pool cleanliness, sanitizers used, or other water chemistry. It is strictly related to the productlmaterials of which the pool is composed.
Once these cobalt spots have begun to form, they become visible to the eye as a dark brown or black crystal, about the size of the head of a pin. If left alone, they slowly grow in size, and they start to develop a “rust” ring around them. If on a vertical wall of the pool, this ring then streaks 1/2″ to 1″ down from the crystal.
To remove these crystals, you can scrape them off with a butter knife or a putty knife. The ” rust” ring can be removed with a rubbing compound, or with a mild muriatic acid/water solution (start with about 1 part acid to 10 parts water, and slowly strengthen if needed.)
For an intermediate-term solution, commercial preparations specifrcally formulated for this problem are available at most pool retailers. They carry a variety of brand names, containing the word “cobalt” coupled with such words as “remover” or “cure”. They are usually very effective in the treatment of cobalting, and most can be used in maintenance doses to prevent further buildup. If the problem is especially severe, standard chelating agents may also help.
The long-term solution is for the pool interior to be re-coated, with newer coatings that usually do not have this problem. This is, of course, an expensive undertaking, reserved normally for only the most severe cases.