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Fastening down sheet metal or even fiberglass panels for exterior applications requires some form of gasket to keep water from seeping under the fastener head. Without a good seal, leaks will eventually cause wood rot, corrosion, or discoloration of interior walls and ceilings. These special washers have an rubber-like EPDM gasket bonded to them which is both ozone and weather resistant. It works well at temperatures up to 350 degrees F, and is ideal for most applications where the primary concerns are ultraviolet light degradation, temperature extremes, and ozone attack. Installation is simple. Just thread the washer onto a Flat Head, Pan Head, or Round Washer Head screw, then fasten with sufficient force to flatten the washer, as shown. Do not dish" the washer though - that can reduce the sealing capability. "