The graphite powder alone will make a pretty runny consistency. The concoction I posted in my first post (#3) will not run. You need to brush it on paying special attention to tight corners.
Here is a little video that I did a few years ago on making and applying a surface with epoxy: Surface Coat
. In the video I used a black pigment instead of graphite powder.
Part-all Film #10 (PVA) works well with Part-all Paste #2 (wax).
Polyester resin (fiberglass resin) generates more heat while curing than epoxy. On thick layups you often have to go in stages to reduce the total mass that is curing. Large masses generate more heat than small ones. Going in layers can help. Laminating epoxy cures slower and generates less potential heat. Typically you can do the whole layup with epoxy in one go.
Epoxy will make a more stable mold than polyester resin. Personally, I only build my molds from epoxy. Epoxy also has a much lower oder level, contains no VOCs, shrinks less, and creates mold edges that are less brittle.
I build a lot of my production molds fromUSC 635 Epoxy
and the medium hardener.