I'm no expert, but I wondered the same thing...I've done a couple and the only answer I have is that I try to push the icing downward to fill in instead of actually smoothing across (like you would smooth frosting on a cake) I also use a rather thick BC. HTH
Mine don't smudge because of the outlining that I do first. Some use thick bc tinted black. I prefer to use canned chocolate fudge icing. The bc I use to fill in has more of a whipped consistency and doesn't crust at all. So it works lovely to smoosh in next to my outline with out actually being strong enough to break it.
It shouldn't smudge if you don't smooth the back with a spatula. Just press the frosting down to flatten it. If you try smoothing the back layer with the spatula when you are done, it will smear your design. I do an outline with a #2 or #3 tip (depending on the detail involved in the image) and just wait a few minutes for it to crust over before filling in the details. Never had any problems. Good Luck!
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