Water leaks in spas and jetted tubs are predominantly caused by leaking pipes in the plumbing system. Even though there may be visible cracks in the surface, they do not normally result in a leak. The glass reinforced polyester resin that is applied on the back surface to impart mechanical strength will normally hold water. However, if water gets to this FRP, and there is un-reacted polyester resin, it can create blisters.

Leaks in a plain bathtub without jets are generally due to structural damage to a wall or bottom surface.