Blisters manifest as round or oval elevations on the surface of spa shells, and more seldom on bathtubs. Originating in the marine sector with gel coat/FRP boats, blisters posed a significant challenge until innovative solutions were developed, which were then transferred to the spa industry in the 1990s. Now, given the vast number of spas produced annually, the incidence of blisters is considerably low.
These defects arise from fluid buildup creating pressure within a void behind the composite’s surface layer, varying in size from small, dime-sized protrusions to larger, saucer-sized pockets, with the most common being the size of an egg. The voids and consequent pressure have various potential origins, and typically, when a blister is breached, it reveals a colored fluid, usually water.
The primary theory for blister emergence in acrylic/FRP constructs is a chemical reaction between water and the polyester resin components, leading to pressure that forms the blister. Water can infiltrate the FRP structure through the acrylic surface or, though uncommonly, via condensation in the application equipment.
The pattern of blisters predominantly occurring below waterlines in spas and their rarity in bathtubs lend credence to the belief that prolonged water contact is a critical factor in their development. While comprehensive scientific studies to confirm the influence of different factors on blister formation are lacking, it’s generally accepted that water chemistry, like pH levels, may play a role in the rate of blister occurrence.
Manufacturers have taken proactive steps to reduce the risk of blisters. These include employing non-corrosive vinyl ester resins for the initial skin coat and utilizing higher-grade polyester resins. Enhanced techniques for spraying and lamination also contribute to blister prevention. Measures such as avoiding air entrapment, ensuring the dryness of the spray system, precise resin formulation, and stringent control over the resin, ambient temperature, and humidity are all critical in minimizing the possibility of blister formation.