Richard Oedel replies: Basic physics here. If you harden the outside layer, then you increase the risk of fracture of that outside layer. The thicker you make the hard layer, the more dent resistant it becomes, but the more visible it is. You can paint the outside surface with CA glue, but that is pretty expensive and pretty toxic with that much exposure. Epoxy will also work to some extent but as you thin it down (using acetone) it becomes weaker.
Another hint, however. I often use MDF for router or shaper templates. It is easy to shape but wears rapidly. So I put thin CA glue on the wear edge of the template thus hardening that outside edge significantly. This allows me to get many, many parts off the shaper.
Garrett Hack replies: Nothing. Use a harder wood with a similar color if you want something tougher.