Did you wet the styrofoam? Can you post a picture of the cake topper?
I have used styprofoam but I'll tell you about my cookie cake. I had a problem getting my buttercream to stick to the cookie at first. Then I decided to take a little bit of the icing and think it with water and i painted it on with a paint brush. Obviously a paintbrush would take years for a whole cake, but I would think you could think your icing down, thinnner than a crumb coat and add that first. Then add your normal layer of icing.
The edges of the dummy cakes are usually very sharp. It is recommended that you either cut or sand down the edges with a nail file. this will give it a rounded edge and won't tear your fondant.
I learned this tip from Sandy Hargrave!
Good luck and hope this helps!
If before you apply fondant to a dummy cake, take the wooden roller you use to roll fondant, to compress the styrofoam all over the sides and top and edges of the dummy cake. This will remove the sharp edges as well as compress the sides and top together and give the foam a smoother look. Use a large paint brush to apply the water and then put the fondant over the dummy cake and fit to the shape as you would a normal cake. It actually works quite well. I hope this has helped you some.
evelyn
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