Are you talking about the cake itself, or once it's iced? If you use shortening, or even butter, it's going to probably do that. Especially if you've covered your board in fabric.
It's the grease seeping out of the icing and onto/into the cake board. Try putting a circle of parchment under the cake before you put it on the board, or cover your board with contact paper.
I buy them from a place that only sells in bulk. I split the order w/ a cake buddy so it's not so much quantity. www.bgpaper.com
Great service, shipped fast, and I love the waxed boards. When I ordered, I had no idea how much I would like them, now I wouldn't go back to regular.
Hope that helps,
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