Fiberglass tanks for power sprayers are manufactured by applying fiberglass and resin around a mold. These tanks are sturdy and long-lasting, and do particularly well in extreme temperatures. They often have a baffle to reduce side to side sloshing. Because the sides are usually flat, fiberglass tanks can also be a good place for additional signage.
Fiberglass tanks for pest control spray rigs often require a steel frame, which adds weight and cost. Some fiberglass tanks are designed so that a frame is not required (see photo). Fiberglass tanks usually require the addition of a fillwell, lid and pickup tube, all of which add cost. These items are not usually required for poly tanks, which come with fillwell and lid and usually feed from the bottom (so no pickup tube is required). These characteristics often mean fiberglass tanks will cost more then poly tanks.
Fiberglass tanks are relatively easy to repair, depending on where the problem occurs. The repairs are effected by applying strips of fiberglass, coating them with the correct resin and hardening agent. If the problem occurs on a seam where two parts of the tank come together, repairs are usually not possible.
NOTE: I will blog the issue of top feed vs. bottom feed in a future issue.
One significant downside of fiberglass tanks is that because a mold is required for manufacture, you can usually only get a replacement tank (that will fit in the space you have) from the company you initially bought the tank from. Contrast this to poly tanks. For example, you can buy a Norwesco or ACE 50 Gallon PCO tank just about anywhere.
Bottomline: Because of their durability, fiberglass tanks are a great option for many applications.