I used a big file that took off shavings but anything coarse enough to take material off should work. You don't really want them sharp or they will chip easy. You do want to smooth the trailing edge. When the rudders are new they have kind of a bulb on the trailing edge so you need to taper the sides of the rudders to the edge leaving just a tiny flat on the trailing edge.
--
/ \ crude ascii representation

that tiny flat should be like 1/16" or so.

cheers
Bill