Mike H: Thanks for referencing this topic. Glad you're going to "go for it" Don't hesitate to reach out and ask me any questions if something comes up.
George. I have a few free minutes at lunch hour so thought I should chime in. I originally found this method on youtube, tried it and was totally sold. I will try to find link to original video, but here are a few quick answer's to your questions:
Like Mike H said, Great stuff regular from Home Depot is what I have had the best luck with. I used the same "LOW LOFT" batting recommended by the gentleman who got me into this.... It comes in Low, Medium and High and refers to the thickness of the batting.....
Here is a link to the original youtube video I watched.... https://www.youtube.com/watch?v=9C6aHEoUWYg
It was done by this gentleman: Scale Model Trains & Colorado's Joint Line
My youtube channel id is: CJ Ambrosi you should find all the making hills and mountains videos there
After I did the first hill module following his method, I decided to add my own twist which was using the expanding spray foam under the batting..... requires much less foam carving time and mess and enables you to shape it as it cures...
I am not a big instructional video guy, but I found this method so much better than making hard shell mountains that I just had to share it.... All my scenery is done this way now, it's cheaper, lighter and much faster in my opinion than plaster and hydrocal.... Once the foam sets up you bury it in paint and start putting down dirt, grass, foliage etc.