I too sometimes have this problem – so disheartening! I recently did a tall cake with 6 layers of sponge and it turned out lovely and smooth. The only thing I did differently was, after filling with buttercream and stacking, I went round the cake with a knife to get rid of the lumpy edges of sponge. I then crumb coated as normal.
Evelyn, NE Scotland, www.facebook.com/Evelyn's Cake Boutique
