Permagrit tools seem expensive but you only really need one the wedge block as shown. This removes balsa and even birch ply with just a few strokes so you can do it slowly and accurately. Also the dust is ccoarser than usual so its a bit cleaner too I think. They are sold in England by Ian Richardson who is often at Top Gun with his magnificent jet models. I was told that his wife actually suggested the design of the wedge block! Very useful to get in tight corners and it is double sided coarse and fine.