I was wondering if anyone could give me their advice on how to achieve sharp edges on cakes with fondant or buttercream. As i look through the galleries I look at some of the fondant cakes and wonder how were they able to get the fondant to get such a straight edge. The same with the buttercream.
all I can offer is - practice practice practice. I'm not much on Fondant making or eating it, but BC, yes. I use a cake "spatula" dipped for a few moments in HOT water, not boiling, just hot tap is fine. Take the "hot" spatula out, wipe it quickly to get the water off, and go around the edges and top to achieve clean edges.
That is soo hard to do! I came the closest just the other day though when I made an ice cream cake. I kept having to pop it back in the freezer... chilling the icing and using a hot knife did the trick for me.
I am extremely envious of those who can get the sharp edges, it's just amazing. 
I don't know if you have seen this tutorial on frosting a square cake yet:
http://www.cakecentral.com/article51-How-To-Frost-a-Square-Cake.html
I use a paint trim guard, but I haven't done a square cake yet since I've had it. It defintely got my cake smoother then I've ever been able to get before. ( I also dip it in hot water and then dry it off quickly beforehand).
I saw that one and it's very helpful... but it's for squares. I don't even have a square pan 
I tried doing it on a round cake and it didn't work for me but your mileage may vary. 
edited to add: I use the putty knives to and it's better than a regular spatula...but that specific technique calls for holding the knife against the straight line of the cake and building up the edge of the icing kind of...
on a round cake it's a pretty small straight line to build up. (not sure if I described that right or even it it makes sense, LOL)
one thing that might help is Ice the top then the sides smooth the sides then the top. That way you can get the sides smooth and what pushes up the edge you can smooth onto the top or scrape off before smoothing the top. Try to keep the spatula as level as possible. Do the same if you use the paper towel method. That may help.
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