A method of manufacturing a plurality of secondary containment panels and assembling and installing the panels in a secondary containment system of an above-ground liquid storage tank or other retention facility includes forming a plurality of flexible substrate pieces. A liquid impermeable coating is applied to the upper surface of each piece such that an edge segment of the piece remains uncoated. The coating is applied to the substrate piece in an indoor enclosure using a computer controlled robotic sprayer that applies a uniform coating. Metal plates are embedded in the floor used for quality control testing of the coating thickness using magnetic gauges. At least a pair of the pieces are juxtaposed relative to one another such that a first one of the pieces overlaps the uncoated segment of the second piece to form a seam.