Yeah, I'm in the "used a piece of leather between the work and the poly board" camp too. Also partially in the "not sharp enough" camp. Once I polished up the teeth on my one set, which sucks and is quite tedious, it worked far better...then I ended up getting Sinabroks and now I just don't worry !
@Frodo, do you mean they used too many teeth on the corner, say, the 4 tooth instead of the 2? If not, I don't know what you mean they used the incorrect chisel so please elaborate on that if you could.