With the master sculpt complete, I gave it a coat of Rustoleum flat gray primer. This was mostly to give the sculpt a surface that wouldn't cause problems with curing inhibition of the platinum silicone I was planning to use. I was careful not to use anything on the sculpt that I knew might be an issue, but just to be safe I put the primer on it anyways.
I used a length of cardboard tube to suspend the mask for molding. Another way to do it would have been to mount the mask on a flat surface with a clay wall around the perimeter. I've seen people mold helmets by suspending them like this, so that's the method I used.
The silicon I used was Rebound 25 from Smooth-On. It's a brush-on platinum-cure silicon, common enough that I can even buy it from my local Blick here in Portland. The first layer was a thin print coat that I stippled on using a chip brush. I used five coats total: two thin coats, followed by two coats thickened with Thivex, followed by a final thin coat to smooth it out. Each layer was tinted a different color so I could see whether I was getting an even coat. One 2-LB trial size batch of Rebound was just enough to give me a final thickness of about 3/8 inch. On each side of the mold I added two circular registration keys to help align the silicone mold correctly within the hard shell.
The two-part shell was made using Plasti-Paste II from Smooth-On. I used a non-sulfur modeling clay to create a wall down the middle of the mold, with a layer of foil to help things peel apart during the demold process. Against that wall I sculpted the first half of the shell, let that harden, then remove the clay wall and sculpt the other half of the wall. After the shell had fully cured, I drilled three holes through the seam between the two halves of the shell for bolts that would hold the shell together.
It's not the prettiest mold ever made, but it does the job. Here's the mold and the original sculpt, side by side.
For my first cast, I used Smooth-Cast 65D from Smooth-On. It's a urethane resin made specifically for rotocasting, so it works well for slush casting in molds like this. I built up the cast in a few layers. The initial "print coat" was brushed into the mold using a disposable chip brush. I tinted the print coat gray so it would be a little easier to see and photograph the details. I ended up with some small bubbles on the surface of the resin cast, which tells me I would need to be more diligent when brushing in that first layer when making the next cast.
Some don't bother with primers on resin. I wanted to give the paint the best possible chance of staying put for a long, long time, so I used a primer. One of the primers that Smooth-On recommends for resin castings is Plasti-Kote sandable primer. It adheres ok, but I was able to get it to chip off with a fingernail if I worked at it. The Plasti-Kote also gives a little bit of a rough, textured finish if you don't sand it down. I didn't mind the extra roughness, so I didn't bother sanding it.
I used all acrylic paints, everything hand brushed. I experimented with a few different brands, including Liquitex, Golden, and P3. For the outlined features (what some may call "cel-shading"), I used an acrylic paint pen. I tried a few different brands, including a Liquitex with a fine felt tip and one from DecoColor with an extra fine plastic roller tip. I ended up using both pens for different areas on the mask. The Liquitex was easier to control on flat surfaces, but the DecoColor was better at getting paint into corners.
After outlining, I drybrushed some of the shadowed areas like under the eyebrows, then I gave the whole thing a blackwash. To seal the paint job, I used Testors lacquer clear coat.