Wouter

By grafite I assume you mean graphite or carbon powder

Adding 5-10% carbon to epoxy produces a hard slick surface.
Adding 30-40% carbon to epoxy produces a soft plastic

The biggest problem with carbon/epoxy is it is messy. You seem to get black powder everywhere.

This was the Gougeon Bros. suggestion for the bottom of a trailer sailed race boat.

I prefer 5% aluminum powder and 5% silica. It is a light grey. It seems little harder and it is not as messy