Paul there seem to be two types, polystyrene or polyurethane. The later is the best for modelling, easier to peel the paper off and takes impressions better as it is a close cell construction.I used plywood profiles, old nylon window screen, & wallpaper paste soaked craft paper for the base & drywall finishing plaster for stone outcroppings on my early RR layouts. I still use the screen & topping compound for crowned roadways today. Weight has not been an issue w/this method nor has cracking/shrinking, even when left in an unheated garage for several years. The last few dios have used packing foam & foam board as the base w/topping compound as a finish. Not thrilled w/foam board. Prolly my fault. PaulE
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