GLI Pool Products 20-1836RE-PRM-BLU 18 x 36 Rect 20 x 39 Blue Safety Cover is constructed utilizing a monofilament fabric that is extruded using virgin resins along with yarns with the highest grade UV stabilization packages available. This time-tested fabric has been engineered to deliver high tensile and abrasion resistance properties for years of trouble-free protection. It also blocks sun, like a solid cover and that means that there is no springtime algae growth.