No reason to ask specific questions without detailed pictures. What i am hoping to find out is how are these compost boards sealed together so water does not get in between the boards.
I'm not quite sure what you mean by "compost board".
If you are talking about composite boards for a deck, decks by design are supposed to shed water between the boards whether they are composite or wood. If you want a deck surface that is sealed and only sheds water to the edge, you would need to consider an outdoor weatherproof surface like Duradeck or something similar.
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