I used to use the hot water trick, but for a long time now, I've used a roller to smooth my cakes. I ice them as smooth as I can with the spreaders, then let them just begin to crust. It has to dry enough so that it doesn't stick to the roller. Then lightly roll across the ridges and you have a nice smooth surface! If it sticks, dust the roller in powdered sugar. My roller has two sides--one straight long side and the other is smaller and slightly rounded. I use the rounded side so that I don't get edge marks.