I have done it in the past using a scraper or a knife blade which is time consuming, and hard on your hands. Ive also used a router table with a 45 degree angle follow bit, but to do this you need to make a template for the router bit to run along and to support the scratch plate. I used 10mm MDF board, used double sided tape to stick the plate to it, and the ran that against the bearing on the follow bit.
You can also use files, but again that can be time consuming, but satisfying.