Remember to put on enough draft for it to release from the sand.
I suggest using Baltic Birch ply, the stuff that has more thin laminations than fir ply; it will give a smooth finish without much hand sanding. Then you need to seal it with varnish.
Sometimes it is a good idea to talk to the foundry people because they might suggest putting extra thickness somewhere on the pattern to minimize shrinkage stresses or voids in the casting. Also if you are machining this all over they may suggest extra thickness so you can clean off any surface porosity.