Making Hills with Expanded Foam
Generally, sheets of Styrofoam® are used to make the basic form of the scenery, and the expanded foam is used to round it out and give it additional shape. Although I do not have direct experience with the two-part foam, I do have some general knowledge to share. Expanded foam uses two different solutions that are mixed together; small amounts of these solutions produce large amounts of foam. If too much solution is made up at one time, the effect is rather like the 1950s movie The Blob, with lots of slithering and oozing. One brand of such foam is Mountains in Minutes.
One specific technique that does not seem to be appropriate for Z-Scale yet does sound interesting for those who have large expanses to cover. I have never tried this, so you are on your own. Basically, the general hill contour is created with chicken wire and wood supports. Once this is properly shaped, spray foam used for insulation is squirted over the chicken wire. While the foam is expanding and generally slithering, plastic kitchen wrap is laid over the foam and chicken wire. It is smoothed out and as the foam cures, the plastic wrap amalgamates with the foam and chicken wire. One brand of this insulating foam is called Stuff-It. This technique sounds interesting but I’m not ready to try it yet.
In any case, once the general hillside is in place, either with cut foam or expanded foam, rock castings are adhered to the support with a scenery cement.