The batsman's Holding......
View attachment 369990
The Bowler's Willey
That's son of....... He probably doesn't even know who Holding is. His dad certainly does.
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To the OP: you're not going to get a long life out of your gutter no matter what you do with the knot. That's just a bit of fast-grown low grade redwood. Even the fact that it was likely pressure treated is irrelevant now as you've cut through the protective surface (chemical treatment only penetrates 3 to 5mm). Your biggest enemy, believe it or not, isn't the water, but the sun, which will twist it and open up the grain, rendering your efforts with bitumen a short term fix. Obviously don't use knotting. In fact, don't use anything. Bitumen is far and away your best answer, but, as I said, even that is doomed to fail in a year or two.