I am sure you found out there are over 130,000 listings on the Internet about Fiberglass Gelcoats, and as you noted, no national manufacturer or distributor is willing to offer a quality Gelcoat for any swimming pool. Your one-piece fiberglass pool, when first manufactured, had a thick Gelcoat placed on the mold first, and then the fiberglass shell on top of that. By manufacturing a pool in this manner, the Gelcoat could not extrotherminate (overheat) and the life expectancy with this application should be about 20 years. A Gelcoat placed on top of a fiberglass shell to be used for harsh swimming pool conditions, will never be warranted by any large manufacturer such as Rohm and Hass, Shell Petroleum, or Interplastics. The life expectancy for this type of Gelcoat application is no more than 5-6 years. Interplastics, who is the largest manufacturer of Gelcoats in the country, will never recommend one of their Gelcoats for a retrofit on an old pool and certainly would never offer any type of warranty.