What you'll need:
- 3/4" plywood
- 1/2" plywood
- chicken wire
- Several cans of "Great Stuff" foam insulation
- Latex paint
- Pull saw
- Nail / staple gun
- Model railroad trees
- Fake snow
Step 1: Build the Frame
I designed the mountain to fit over the existing trackbed and provide a tunnel for the train to go through. For the base, I simply cut a few pieces of 1/2" plywood. To help support t he chicken wire and to add to the height of the mountain, I cut ribs out of 3/4" plywood and screwed them to the base.
Most important was testing the train and track to ensure it would fit through the tunnel without scraping the side.
Step 2: Wrap With Chicken Wire, Stuff With Newspaper
Once the shape of the mountain is complete, stuff the form with newspaper. This will give the spray foam something to stick to. Use as much newspaper as you can and don't worry if you cover the space for the tunnel -- you will be removing the newspaper once the foam dries.
Step 3: Spray Foam
I ended up using "Great Stuff" spray foam intended to fill small (1-2") gaps. I tried the foam for larger gaps, but it didn't seem to make much of a difference. In fact, I thought it was harder to work with. Each can cost about $2.50 and for the size of this mountain, I went through 7 cans total.
Hold the can close to the wire frame and begin applying it in rows. The foam will expand slightly after application so you don't have to pile it up. You will need more cans of spray foam then you originally estimated. It looks like it goes a long way, but unfortunately does not. Keep in mind that once you open the can it will need to be used completely.
The outside of the foam will harden in a couple of hours and become completely solid in about 24 hours. Don't rush it --- you don't want to cut into the gooey center.
Step 4: Sculpting Your Masterpiece
I used a fine-toothed pull saw to carve the faces of the rock. It worked well because it was long enough to cut wide faces. Work your way across the entire mountain cutting off the rounded "lava-like" sections of spray foam. Be careful not to cut too deep -- you don't want the chicken wire to show through.
I cut horizontal and vertical ridges into the rock face to simulate a real mountain.
Step 5: Paint
The first coat of paint helped to highlight areas that needed work. I used an additional can of spray foam to fill in holes and remake areas that didn't look quite right. After the foam dried, I used the pull saw to trim off the rounded areas.
Using standard acrylic paints (from Hobby Lobby), I mixed up some dark grey and brown colors to feed through my airbrush. The first couple of passes didn't look too good -- the mountain looked as if it had a bad camouflage pattern. It took several coats before it started to look like actual rock.
One technique that I found useful was to "force" light and shadow. That is, I applied a lighter color to the rock faces that were more horizontal and a darker color in the crevices and vertical slopes.
Many model railroad websites recommended watering down the acrylic paint and then applying with a spray bottle. Unfortunately, that technique didn't work well with this type of foam as it was not as dense as hobby foam.
Step 6: Finishing Touches
I found some snow covered trees that my wife had purchased years ago for a Christmas village, removed the bases, and stuck them into the foam. I also placed trees along side the mountain to round out the effect.
I hope this Instructable has inspired you to run off and move your own mountains. If you have any questions, post them in the comments and I will happily answer them.