Capstock composite decking basically consists of a protective wrapper around a core made up of a blend of wood particles and plastic resins. Because the uncapped ends of the boards are free to pick up water, the core can absorb moisture, which can lead to all sorts of problems. To help prevent swelling, I follow the manufacturer's recommendations for gapping butt joints, and I also take an extra step. Just as with installing cedar, Ipe, or any other wood decking, I seal the ends of composite decking with lumber wax. It makes these types of decking resistant to water absorption and therefore fungus and swelling. While there are a variety of sealers, I use Anchor Seal because that’s what my retailer sells.