I've used styrofoam dummies many times for show cakes. First, I either file down or press down the top and bottom edges so they're not so sharp. If there are any big holes or chunks out of the styrofoam, I filled them with royal icing. For buttercream cakes, I just frosted the cake just as if it were real cake. For fondant covered cakes, I spread a thin layer of shortening on the stryfoam, then covered it with fondant with no problems.
If you need to carve, use a serrated knife or better still, use a hot wire cutter made for cutting styrofoam. They can be purchased at craft stores. Hope that helps.
KimAZ
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I competed on Ultimate Cake Off- Monster Jam!
I competed on Ultimate Cake Off- Monster Jam!