____Perfectly Perfect No Border Showin Cake!____
For simplicitys sake, I am using a round cake as an example when Im describing the method. Same thing basically applies to square cakes though too.
You are using an extra board for this method of smoothing. So your cake is sitting on its board/circle/foam core by the time you begin the method Im going to explain. It is best, for this method, if you use a circle that is exactly the size of your cake, or slightly smaller.
After filling, crumbcoating, whatever you do, you place your cake on another board that is from 1/8th inch larger in diameter to however much larger you want it. All depends on how much icing you want on your cakes edge.
I have a masonite board for each and every tier size I offer. For example, for my 6 rounds, I have a board cut from masonite that is 1/8th inch diameter larger than the foam core board I place my 6 cake on.
This masonite is not the thick type that you cover with fondant or butcher paper for final presentation, this is a thinner, flexible type, at about 1/8th inch thick. Its smooth and glossy, and is probably called something else, but Im calling it masonite. Either way, Im sure its not hard to figure out.
So, you have secured your cake and its usual board to the larger one weve been discussing. I use a bench scraper that has no handle, exactly like the one you will find by Googling Progressive Bash and Chop. This can be found at Bed Bath and Beyond. Wherever you get it, whatever brand, no matter. The important part is that it be able to sit completely flush against the turntable AND the larger board. You will be smoothing by following the larger board.
When youre done, pop the whole thing in the fridge. Wait a couple of hours until it is firm. Take it back out, jimmy the larger board loose, slide a teeny spatula under it, whatever you want to break the cake loose from the larger board.
Admire your perfectly perfect cake, with not a smidge of border showing, because if done right, you have completely hidden it by using a second board to ice against. Wash up the larger masonite board, and store for future use.