Sooner or later the splining will deteriorate, and whatever sealant you put in may as well. Also, you are creating little pools in the gaps that may be breeding grounds for mosquitos in wet weather. Have you considered a waterproof underlayer instead of sealing the seams? You could use any of various roof
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Expert advice on deck waterproofing systems that protect the area beneath an outdoor deck from rainwater. They lock together along their long edges to form leak-proof joints. Dexedry flanges are available in a number of sizes that match the edge grooves found on major synthetic decking brands.
For cleaning, I highly recommend using a power washer on composite decks as opposed to just a garden hose and a scrub brush. A power washer will not only help get things cleaner than scrubbing by hand, but it will also do it much faster. Just remember, you can't remove anything left on the deck once the project is