Hi Artur, I know that miters of all kinds, including that kind, are often cleaned up using various kinds of miter shooting board. I am not sure, but the shooting board for that sort of joint might be the donkeys ear. Some time back I saw an article somewhere by Paul on different kinds of shooting board. And a while ago I watched a youtube video about the many sorts of shooting boards for various miters. A search for “miter shooting board doneys ear” on youtube will probably get you the vid I saw and others.