The hardest part about this whole process is getting the Excalibur plaster mixed right. It takes a bit more dry powder than you realize. The best way to tell when you have added enough plaster is to sprinkle in just a bit and see how fast it turns dark grey as the water soaks into it. The texture when you have enough water added reminds me of a slightly melted milkshake.
Once you get the mixture down casting Warhammer 40k terrain out of a silicone mold is really easy. Just pour a bit in the mold, give it a tap, and then top it off. You want to over-pour just a bit because what happens is as the Excalibur dries a layer of water will rise to the top and settle. After about six minutes after pouring the plaster you will need to soak this off with a paper towel. Once you have done that flatten out the mold using a putty knife and then give it about thirty minutes to cure.
Excalibur plaster puts Plaster of Paris to shame. The PSI rating on Excalibur is about four times as strong and it holds paint and glue a lot better. For me the cost has been well worth it (the Excalibur was about $70 shipped for a fifty pound box - that is a LOT of plaster) and I plan on making a good deal of durable scenery and terrain out of it.
Here is the final piece - all painted up and sealed. The paint and sealant on this piece goes a long ways to toughen it up. If you run your fingernail on the rough part on the bottom it doesn't scratch or flak off. With the plaster of paris it would just crumble.
I have four of these pieces cast and painted up. I will probably make a few more and then move on to something else. Anyone have any ideas on what my next piece of terrain should be? Let me know in the comments.