If you're not familiar with how Ultrcal (or really any gypsum material) and latex work together, the Ultracal actually sucks the water out of the latex, enabling you to create a thin skin. The longer you leave the latex dwelling in the mould the thicker this skin will be. I my case I need just a thin enough skin to protect the Ultracal from the polyfoam, normally you'd want enough latex to totally fill the mould to create something thicker.
I only have a little onhand so I just did a quick brush on coating to ensure it was evenly coating the deeper recesses on the front of the face, joined the halves together and sloshed around more latex, probably about 500 ml at this point. The latex was drained and I usually paint on more around the opening of the pour hole (again to prevent the expanding polyfoam from biting into the outside of the mould).
The polyfoam will normally adhere and lock into the Ultracal and it's a bugger to paint as well, hence the whole latex skin. I was using a very quick expanding and curing polyfoam (from Monster Makers) so I had to be quick about it, I made the pourhole larger than normal to make sure I could pour the foam in fast enough. It worked perfectly.
Also, since it has a very thin surface of latex, which keeps it really flexible and tough, and the expanding polyfoam is really low density, these heads are incredibly light and pretty squishy as well, so you can throw them at people without hurting them - if you ever feel the need to throw a severed zombie at someone.
So that's pretty much it. I can make several dozen heads from this, possibly more if I'm careful with the mould so the polyfoam doesn't ruin the moulding surface.
While only about a dozen are needed for a couple scenes in the series and this mould should last for awhile, I'm planning on making these available through the Funding Campaign for about $40, I'll only be making 10 of these for the campaign - more on that later.