A friend lent me a cast resin retaining wall he had used for his HO waterfront railroad. It's a long wall and the stones are large enough to use it for O scale. So I mounted it on a sheet of Plexiglass and made a mold from it. I used Micro Mark 10 to one casting rubber. I chose the rubber because of it's high strength and its ability to resist tearing. It should give me dozens of plaster and resin castings without showing any signs of wear.