I've made 2 buttercream camo cakes. I do not ice first. I just have 4 bags of colored icing ready to go and squeeze fast. If I need to smooth parts out as I go along, I use a spatula. You just have to go pretty quickly. BUT, if you can't get the whole cake covered before the icing dries, I would imagine you can Viva the crusted parts only...a little at a time, instead of the whole cake at once.
It's really not hard. I was a little intimidated about doing it at first, but found it was much eassier than I thought it would be!
Good luck!
Here are my two camos. I made the tank one first. The square was after I had a little more experience.
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1399848
http://www.cakecentral.com/modules.php?name=gallery&file=displayimage&pid=1194962
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