The local box stores or your hardware store should have, car bondo, or wood bondo. Basically the same thing. You want anything that is non shrink. The issue is that you will need to watch it every year, and most likely fix it every once and a while. There is no magic solution, accept changing the whole board.
Sorry about the 72 views, I can't not stop looking at an unanswered post...kind of like a nascar wreck where you can't look away.