Use a smoothing tool for drywall. is much easier then the spatula. I don't use bettercream, but when I use my non crusting, I do it in layers and cool it in the fridge between layers for a few hours. Its much easier then trying to smooth out real thick buttercream...you have to do it alittle a differently then the crusting kind.
If you are having difficulty smoothing Bettercreme, or any of it's counterparts, one of two things may have happened -
1 - You overwhipped it. In that case, you can put a couple of tablespoons of the liquid into the bowl and rewhip it, just until it's incorporated and soft.
2 - You are not working with it fast enough. It only stays soft as long as it stays cold. Once it comes to room temperature, it turns sponge-like when you try to smooth it.
I've only worked with that stuff once, and hated every minute of it. But I can still tell you what I learned. I had asked, the person who
recommended it to me, how to smooth it. She suggested a hot knife, like you would use with SMBC. It only made things worse. Then I realized, it must stay cold to stay soft, so I dipped my spatula in ice water. It worked like a dream! It softened back up and smoothed out perfectly.
I still hate the stuff, but if I ever absolutely, positively have to work with it again, I will know what to do.
Theresa 
Just remember to wipe the water off the spatula before you use it on the cake. The last person I taught that didn't put 2+2 together, and had streaks running down the side of the cake. 
Theresa
Thanks everyone for the tips! I ended up taking it all off the cake and making my regular buttercream. That's the last time I try something new on an order. I have to say I do really like the taste of the bettercream and want to try it with different flavors but will only use it on cupcakes. I really needed my frosting to crust over. I am very picky about my frosting being smooth on my cakes
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