May 9, 2017 ASK THE BUILDER Composite decking is a nice alternative to natural products that need frequent sealing. The cracks get wider and deeper, and eventually the treated lumber starts to resemble a 50-year-old fishing pier. Even the best deck sealing products don't hold up well against the harsh UV light.
Home Depot. /p/NewTechWood-DeScratch-Composite-Decking-Repair-Kit-in-Peruvian-Teak-DR-TK/205405755. 0 · Reply. Vernette Sedgwick, CO. on May 13, 2015. Behr has a treatment / sealer called Overdeck. It has a rubber and sand base. It comes in colors. It will fill-in cracks and protect it.
Have you considered a waterproof underlayer instead of sealing the seams? You could use Caulking between the composite deck boards works if sloped right. I sloped If you caulked that, it would work like a champ and small cracks would drain down the channel (inside the board) to drain. My idea.