The styrofoam I use for carving usually has bits of loose styrofoam, so I always clean it and then do a "crumb-coat" to seal in all of the loose pieces and create a smooth surface. I let the icing harden completely and then brush lightly with water and apply the fondant. Afterwards I can peel the fondant off without pieces of styro sticking to it.
I also use a layer of BC under the fondant on styrofoam.
No need to cover with anything before applying fondant.
Brush off bits of styro using a soft pastry brush. Massage in a nice bit of crisco. It will fill in lots of imperfections. Cover with fondant.
HTH
Rae
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