For razors I use clean flax and horsehide. For knives I use a 3 x 3 x 12 balsa block. I have white, green and diamond spray as well as one side I glued a piece of veg tanned leather. Balsa is soft I agree. But it keeps you honest. It's easy to see if you are making mistakes. Plus it's easy enough to sand it flat if you dent it. Or if you decide to use some other paste.