I have been working on tunnel walls this past week. Using a kitchen knife, I split a one inch piece of styrofoam. As I work the knife around and through the panel, I cut and twist the blade causing the styrofoam to fracture and break apart.
The results produces two 1/2" panels that resemble a rough rocky structure. The panels are then painted a dark earthy color. Spray with "wet" water (add drop or two of soap) helps to bleed paint into cracks and crevices.
The panels are easily bent to follow the curve of the track in the tunnel area.
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Really clever and creative. Plus, your technique allows anybody who uses your method to further grime and weather-up the walls as their personal creative spirit and experiences move them. You've made a nice contribution, here, Darlander.