Smoothing the Sides and Top: Smoothing the Sides with a Bench Scraper
Okay. Your final coat's on from last time. It's not smooth yet and that's fine, it's not supposed to be. Tonight's the part where it actually starts looking like a cake somebody bought a bench scraper for.
If you don't have one, get one before tonight if you can. A metal bench scraper, straight edge, no fancy angles. You can improvise with a large flat spatula in a pinch but it's harder to keep steady and I won't pretend otherwise.
Set up your turntable
You want your cake dead center on the turntable. If it's off to one side you'll be fighting the wobble the whole time instead of fighting the frosting, which is the fight you actually signed up for.
Have a tall glass of hot water next to you and a clean towel. You're going to dip the scraper in the hot water, wipe it dry-ish on the towel, and use it warm. Warm metal glides through buttercream. Cold metal drags and tears. This trips people up every single time because it feels backwards — you spent all this time chilling the cake and now I'm telling you to heat up the tool. Both things are true at once. Cake cold, tool warm.
The actual smoothing
Hold the scraper straight up and down, flat against the side of the cake, at close to a right angle to the turntable. Rest your other hand lightly on top of the cake, near the edge, just enough to steady it, not enough to push down into the frosting.
Spin the turntable slow and steady with one hand while the scraper stays still against the side. You're not moving the scraper around the cake. You're moving the cake past the scraper. That's the part people get backwards and it makes a real difference in how even it comes out.
One full slow rotation, then stop, wipe the scraper clean on your towel, dip it warm again, and go around once more. Don't try to fix everything in one pass. You'll take off a little more buttercream each time and it evens out over three or four rotations.
If you hit a gap where you can see cake through the frosting, stop. Don't keep scraping over a thin spot hoping it fills in, it won't, you'll just tear it worse. Take a small offset spatula, add a bit more buttercream right there, let it sit a second, then go back to the scraper.
Cleaning up the top edge
Once the sides are even, you'll have a little collar of extra frosting standing up around the top rim, like a lip. Take your scraper, hold it flat and level, and drag it across the top toward the center from one side, just once, to knock that lip down flat. Don't saw back and forth. One clean pass, wipe the scraper, turn the cake a quarter turn if you need to come at it from another direction.
I'll be honest with you here since I'm honest about everything else — I still can't get a truly sharp edge on that top corner the way you see on the professional cakes, the ones that look like they were cut with a ruler. Mine are good. They're not razor clean. I've made peace with it. If yours comes out good and not razor clean too, you're right on track with me.
About the scraper itself
I have one bench scraper that's bent at exactly the right angle from years of use, and I genuinely guard it. Someone at a ward function grabbed it out of my kit to serve beans off a chafing dish one time, no idea why, it was just sitting there and looked useful I guess. I rewashed that thing three times before I'd touch cake with it again. So — get your own, learn its bend, and maybe don't leave it sitting out at potlucks.
A word on patience here
This is the same opinion I've had all course: warm buttercream and rushing are why cakes look like accidents. If your frosting's sliding around and smearing instead of smoothing, it's too warm, full stop. Pop the whole cake in the fridge for ten minutes and come back to it. Don't keep pushing through a mess hoping it resolves. It won't.
Also — wipe your scraper every single pass. A scraper with old frosting caked on the blade will drag lines into your clean surface and then you're chasing your own mistakes around the cake. Keep that towel busy.
Before next time
Get your sides smoothed and cake back in the fridge for a short chill once you're happy with it, we're doing the top surface and edges properly next time and it goes easier cold.