POLYTAPP Skylight Fasteners CORROSHIELD® POLYTAPP® is designed with a wing feature to grind an enlarged hole in the skylight material to allow for skylight sheets movement under weathering effects.   This allowance will prevent skylight sheets from damage as a result of stress and fatigue due to thermal expansion and contraction. POLYTAPP® wing feature is also designed to break off automatically with it comes into contact with the steel substrate. The large diameter dome washer will also provide that perfect water proofing.