I've had a lot of success spraying Elmer's yellow glue diluted about 2/3 or more with water. For some applications, particularly ballast, I've found the sprayer to be much more effective at not disturbing the loose scenery material and thus preserving its shape. A mixture with only 25% - 30% glue is still plenty to hold most scenery material in place, but if you want more strength just wait until the sprayed glue dries, and then apply a thicker mixture dribbled from an old glue/ketchup/whatever bottle. The original shape will still be maintained.
For base scenery applications other than ballast, I apply the same glue mixture directly to the plaster shell with a brush. Final details are secured with the sprayer.
