One of the best trimming tools I own is a Bison trimming tool. You can purchase them from the maker or on ETSY. They are spendy at around $60 and you cannot drop them.....they are made of titanium and will break.
Because they are very sharp, sometimes they will "drag" on the clay. I then use a rasp or blade from a sheetrock finishing tool to roughen up the surface. This is the ideal gift for an advanced potter.