Step 3: Covering the Sculpt with Plaster
Notes about plaster:
I'm using plaster of Paris for its low cost and ease of access. It can be a little "soft", and not too durable for detailed work like this. A stronger plaster like hydrocal is usually used for this process. If you can afford the cost, go for this higher quality plaster. But for cheap DIY projects, plaster of Paris should work just fine.
Mix some plaster with water to a thin consistency and brush some of this onto the sculpt, ensuring to completely cover small details, like wrinkles or ridges. This is called a beauty coat. We cover these delicate details first to make sure they are properly transferred to the mold. After that, you can mix a thicker batch of plaster and spread it all over the mold. I just used my hands. Tap the head a bit to get some air bubbles out.
To get more plaster under the chin, wait until most of the plaster is somewhat dried, and then turn the head upside down and cover the lower parts of the head. You'll want at least 1/2 inch of plaster covering the whole sculpt on the first layer.
To build up the mold, I used alternating layers of plaster and plaster strips; I'd lay on a layer of plaster, and then add a layer or two of plaster strips. I knew that plaster can be fragile by itself, and so these plaster strips were a good way to add durability to the mold. Add more plaster, then more strips. Repeat. I did this until the mold was over 1 inch thick. Then let it dry, for at least an hour or two.
When disposing of excess plaster, do NOT pour it down the sink; it can harden and clog the piping.