Mercury - SmartCraft Weather Cap Junction Box Protective Cap 859318T2. Overview. The Mercury SmartCraft Weather Cap (859318T2) keeps your SmartCraft junction box protected from water, salt, and corrosion. It seals any unused connection points on the J-box, keeping the system clean and dry. Simple, small, and built to last - it's a must-have for any SmartCraft rigging setup.. Key Features. Weather Protection Cap - Covers open SmartCraft junction box ports to prevent corrosion and water intrusion.