What are your shells made out of? Fondant or chocolate? If they are molded chocolate you can take a small knife and scrape the edges, then take your finger and rub the edge to smooth it out. The heat from your hand will melt the chocolate just enough to give you a smooth edge.
Maybe you're icing is too think. Is it hard to pipe? I know that I have that problem if I get too much Powdered Sugar in my icing and it's too thick. You might try thinning it a bit if that 's the case. You might also check your tip, if the edges are jagged, it will come out that way when you pipe.
Maybe some of the other CC'ers have other suggestions, good luck.
Your icing might be too stiff, I have that happen with my roses...
I guess I'm confused -- are you talking shell borders piped with buttercream? Or are you talking molded seashells? I thought you were talking molded seashells (my students usually say "shell border" if that's what they are struggling with.)
OH! Okay, I misunderstood. Anytime you pipe with buttercream, no matter what you are piping, if you have a jagged edge it means your icing is too thick. Thin it out starting with 1 tsp per batch until it gets to a workable consistency.
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