A crumb coat is a very thin layer of frosting that helps prevent any loose crumbs from turning up on the outside of the cake. Once the layers are stacked with any desired frosting or fillings in between them, use an offset spatula to spread a thin layer of frosting all over the outside. When dipping the spatula into the frosting bowl for more, first scrape it off in a separate small bowl to prevent loose crumbs from getting into the frosting. You can reuse any crumby frosting from the separate small bowl while applying the crumb coat. Use a bench scraper to smooth the edges (see next step) and then place the cake in the freezer for a few minutes before continuing on to the final layer.