Most manufacturers use the fiberglass lay-up method of spa construction. This manufacturing method combines layers of polyester resin and glass fiber. A heated acrylic sheet is placed over a spa-shaped vacuum mold. Air is drawn out through hundreds of small holes. When the acrylic cools, they remove it from the mold and reinforce the underside of the spa with a resin and chopped fiberglass mixture. This polyester resin and glass combination can lead to a common problem—acrylic blistering. Blisters occur because of a chemical reaction between moisture from the tub and the polyester resins used in the reinforcing process. Many spas have been ruined because of blisters. Spas made from polyester resins are not blistering resistant. Their finish and structure warranties are separate. The salesman points out the structure warranty because it is usually 10–15 years. Many times he won’t mention that finish warranty is only 1–3 years. You could have a blister problem in as little as one year! Great Northern® Hot Tubs don’t have this blistering problem.