End grain cutting board issue
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I came across something unusual making an end grain board today. These strange wavy lines that appear only at the top and bottom of the board, and look identical on the backside. It was fine coming out of the drum sander, and it was only after round one of sanding I noticed it. At first glance it looks like a sanding issue, but no amount of re-sanding would correct this. And the rest of the board is fine. At a closer look, these lines are oddly symmetrical with each other, front side and back, so seems unusual to only have a sanding issue like this. I don't think I could be that consistent making unintentional wavy lines so similar. My thought was some kind of compression damage from the clamps after glue up. Clamps were placed where the yellow Xs are, 3 underneath and 2 across the top, which kind of line up with the waves. But again, it looked fine after drum sanding. After that, the left and right sides were trimmed off with table saw about 1/8 inch. I know a lot of comments will point to sanding, but this anomaly goes straight through to the bottom of the board and looks identical on the other side. Thoughts?