There is no need to undercoat every cake. But there are many than benefit from an undercoat. Any cake that has a lot of crumbs on the outside edges has to have un undercoat so the crumbs are trapped in the undercoat and do not show through on the finished cake.
Typically, carrot cakes, banana cakes and others finished with a powdered sugar based frosting do not need an undercoat because the cakes do not crumb much and also because these types of frostings can be put on thick enough one step. However, if you have any question, always apply an undercoat.
While it is an extra step to undercoat, sometimes referred to as a crumb coat, the finished cake will be so much better looking. This is particularly true for cakes finished with Italian or French buttercreams or a ganache. Ganache will follow the side of the cake very closely making for a rippled effect on the sides. Continue reading