I just completed my first fondant cake with milk chocolate ganache underneath (since I hear ganache makes it much easier to get sharp corners from fondant) and had mixed results (see photo below). I LOVED the sharp, perfect look of the ganached cakes BEFORE covering with fondant, but I experienced some of the same issues applying/smoothing the fondant as I did using SMBC underneath.
Even though I was careful to brush the entire cake cake with a hot syrup mixture, I still had a lot of air bubbles come up. I think one cause of this may have been that the fondant began adhering to the syrup on the sides of the ganached cake before I'd had a chance to spread it's "skirt". Is that supposed to happen? I also used pieces of thick acetate to sharpen the corners and smooth the fondant, but I still had issues with the edges. The photo doesn't allow you to see it super well, but the top tier had a little round bulging at the sides and the corners of the bottom tier seemed to sag during smoothing. I ended up getting a bit of a protruding, lowered rim/lip where there should be a sharp perpendicular corner. I think another possible cause is that the ganache wasn't firm enough. I used about a 2.7 to 1 ratio for milk chocolate to cream, but next time I think I'll use 2.9 or even 3 to 1 (like white chocolate).
This is only my 3rd time covering a cake with fondant, so I know I shouldn't be too down on myself but with all the gorgeous perfectly sharp corners I see on here and Facebook all the time, it's hard not to be frustrated when you spend so much time and the results are still so flawed. Would love any and all feedback about how to improve my next fondant cake. Thanks!
I use ganache under all of my cakes now. I got the Planet Cake book, which gives lots of instructions and info on it.
I've never used hot syrup to adhere the fondant to the ganache. I either spritz a little water on the ganache, or if I've left the cake sitting out long enough, I don't use anything. My ganache gets tacky by itself if it sits out and warms up on the cake a bit, and it that happens, I just put the fondant right on the cake. Like you, I still want my cakes to have sharper, prettier edges.