I think I have used every possible product, technic, suggestion, idea, or combination to model mountains and terrain. On my latest layout I am using a one, two, three procedure that works great. Bacic form is Great Stuff, which is then covered with plaster cloth and finally painted with Flex paste. I prefer Poly Terrain but they have vanished??? Cheaper, better stuff. Thankfully Hobby Lobby had Flex Paste at 50% off after Christmas and I stocked up. While you can't cast Great Stuff, you can control the final result by how yo add the plaster cloth. You end up with a very light weight terrain that takes whatever scenery you desire to apply.
Doc