i usually either use a super thin layer of buttercream, or airbrush the cake board lightly with plain water, or I've also used pipping gel.
HTH
You can use piping gel or water.
Here's a previous thread that might help:
http://cakecentral.com/cake-decorating-ftopict-625978-fondant.html+board
In this thread you will find a link on the bandage method that's really handy.
HTH!
Ummmm, are you meaning to use this fondant covered cardboard as your cake base? Like a cardboard round meant to go underneath individual cake tiers? If so, that is going to be unbelievably unstable and will bend and crack and may cause your cake to shift and crack and fall and do all sorts of crappy things.
If you mean a cake drum or foamcore or masonite, then...whew! Just checking, don't want you to experience cake support failure!
She uses water.
http://www.flickr.com/photos/extra-icing/sets/72157594575839118/with/413921673/
She uses piping gel.
http://www.cakejournal.com/archives/how-to-cover-a-cake-board-with-rolled-fondant
But I know that for other things she sometimes uses tylo glue.
http://www.cakejournal.com/archives/how-to-make-tylo-glue/
And I've heard that you can use diluted royal icing.
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