When it comes to knives and scrapers there is a large amount of difference in the ones that are available. The fundamental difference between a filling knife and a scraper is the flexibility.

With a filling knife you get the flexibility which allows the filler to be forced in to the hole. A scraper has no flex so that it can be forced across a surface.

With a scraper you push it across the surface and with a filling knife you pull across the surface. This will protect the edge of the filling knife and achieve a very good finish.