Hi, they are popular with the trades but go under a variety of names and the search engines seem to have trouble finding them because they autistically assume I’m looking for the conventional handled filling knives decorators use. I’ve seen them called Japanese filling knives, Continental filling knives, Square filling knives, Flat filling knives. I knew they were good when I borrowed one from a painter and he became concerned about his favorite tool returning - and they cost about a euro each so I bought a dozen packs when they hit the Lidl store.
The best ones I’ve found are from the value store Lidl where they go under the name Profil Powerfix Japanese Filling Knives. They are inexpensive and are the ones resold online with the red plastic grips.
These rust easily but are easy to maintain with a brief encounter with sandpaper - they also get better with use. I bought some stainless steel versions but they’re less effective and five times the price. I use them on cleaning up difficult jobs like removing concrete from door sills and window tracks, removing plastic and paint behind hard to reach door hinges - and of course cleaning up window frames and squaring the frame edges without wall damage as I described. They’re one of those tools that when you know how to use them you cannot imagine doing it any other way - I think most window cleaners use putty knives, screwdrivers or window blades where I’d use the filling knife but the problem with every one of those tools is that they can score the door and window frames, tracks and sills - too sharp or too pointy - and those tools are slower.
