Making ROCK

Back In November 2009 I made a very large batch of rocky terrain features, including a chasm, some cave pieces and badlands. Due to the cold weather setting in and the large volume of pieces required for these projects, I opted to avoid using my hot wire cutting tools, as the fumes are difficult to exhaust from my basement studio. The Cave pieces were developed by reusing scraps of foam from previous projects, which gives them a nice random appearance and allowed me to clear out some scrap from the supply yard.
A few observations on gluing EPS foam: I used wood glue to adhere the layers of foam used to make the chasm, with toothpicks to reinforce and hold them together. I had to weigh these pieces down in a large stack with a stump, and even after drying for several days, the wood glue was still damp or wet in places. I have since switched to using two or three hot glue guns for large pieces, which necessitates working quickly. There are other options, such as Liquid Nails, but I have found hot glue to give the best and quickest results so far, and it is less toxic.
Another observation about EPS foam is that is not perfectly formed, or perhaps becomes warped during storage and transport. What that results in is the surfaces of two flat sides not being flush when pressed together. In the future I am looking in to finding a good source for larger, thicker blocks of EPS to make the cliffs and chasms with, so that layering will not be necessary. Any excessive gaps were filled with a vinyl spackle.
During the sculpting the foam I experimented with some new tools and techniques as a result of opting against the hot wire cutters. The chasm pieces were textured using a power drill, by running the side of a flat bore bit along the face of the cliff. The bore bits can also work for drilling or digging deep recesses with great results. Other tools used to create contour and shape were a metal sculpting rasp and a wire brush, in addition to the usual box cutters, sheet rock saws and bread knives.
PROPANE!! A propane torch was the final element in forming the rock pieces. The rough shapes required a bit of smoothing, to add a weathered, eroded look and melt off some of the debris and lighter scraps still clinging to them. I took the pieces outside and very lightly ran a propane torch over the surfaces to slightly melt and shrink the foam a bit. I used EXTREME CAUTION to avoid creating or breathing the vapor and fumes. If you are careful enough, the foam will not be burned, and very little hazardous fumes will be created. I used a filter mask in any case, and the wind was also helping to clear fumes away during this process. I would NEVER advise using the propane torch for this method indoors.
The painting process was a big production. I set up some large tables by the grill and base coated everything in a couple of days with Interior matte latex paint. This is durable and very economical, and the best solution for large quantities of terrain features.