I believe the issue with "self skinning" is due to the fact that it self skins where it's not touching anything. I don't believe the self skinning is going to work up against a mold. Check the leftover in the cup you mixed, I'd be willing to be that part looks more like what you're hoping for. I'll try to remember to ask my guy next time I go in to pick up materials, they experiment with a lot of stuff and may have come up with a solution.
I would think you would want to look for a flexible coating you could brush on to hopefully cover the pin holes? I don't know.... just an idea.