Hi Jenie
I agree with SaltCakeCity, Fondant is so sensitive to moisture. Whipped cream is best used only between the layers. The fat in the buttercream acts as a barrier - therefore keeps the moisture from the cream in. I would place the cake in the fridge to chill for an hour or so after filling it, and again whenever you coat with buttercream. - to FIRM up. Chilling makes the cake more stable and much easier to cover with fondant.
Hope this is of some help!
Margaret393