Plastic (acrylic, gel coat, etc) spas and bathtubs are normally produced as a composite product. It will consist of a surface material to provide good aesthetics (sometimes as two layers) and a reinforcing material to provide the mechanical strength of the product. In addition, other materials, such as wooden supports and polyurethane foam, are often used to add strength to the product. When the integrity of these reinforcing materials is sacrificed, the structural strength is reduced.
This frequently requires procedures and materials to re-build the structure.