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[markbc]

You can make a hard coating using a 1:1:1 ratio mixture of white portland cement, fine silica sand and mineral oxide pigments on a bondcrete paste coat and applied to the polystrene which turns as hard as rock when it dries.

It makes a very realistic looking rock wall with a rock textured finish to the final wall. It looks more like sandstone and you can use different oxide pigments to get different colours.

This method also prevents cracking provided you do not use much foam filler.