Fiberglass is increasingly used for corrosion-resistant industrial applications. Fiberglass laminate typically consists of a structural layer and a corrosion-resistant layer (commonly referred to as the liner) on the inside of the laminate. Liners are used in all corrosion-resistant fiberglass applications to guard against corrosive chemical attack on the structural layers of the fiberglass. The ability to measure fiberglass thickness from one side of a laminate during equipment operation quickly and easily with high measurement accuracy is extremely important for life assessment and prediction. Microwave nondestructive techniques employing open-ended rectangular waveguides in conjunction with robust and full-wave electromagnetic models have tremendous potential and viability for evaluating complex layered dielectric composite structures for the purpose mentioned here. In this paper, we describe the foundation of this technique, processes involved in optimizing the approach for layered fiberglass evaluation, as well as a number of representative and diverse measurement results demonstrating the efficacy of the technique and the thickness accuracy that can be obtained. In addition, other important issues such as the effect of curvature of the structure and water with different temperatures filling it were also addressed and shown not to adversely affect measurement accuracy associated with liner thickness.