Step 3: Spray Foam

Important: Apply the foam in a well ventilated area!

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.
Are you sure that the recommendation for thinned acrylic paint was for foam mountains? Or was it for foam mountains covered with a layer of plaster. For example, using plaster gauze used for casts as in a broken bone? Or plaster coated paper? Or a coating of plaster spread with a putty knife? Or even plaster cast in a rock mold?
Awesome Instructable! These techniques are simple,effective and usable on many unrelateds. My only "criticism" (way to strong a word, as I really think it came out EXCELLENT) is there is no portal. Typically a tunnel has a concrete or stone block structure at the entrance and exit.
Thanks for the tip -- I suppose I could carve a portal out of wood or perhaps form it with Super Sculpy.
amazing job love the whole track did you know you could add a bit of realism to train by weathering
Thanks for the recommendation!

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Bio: Mad scientist, woodworker, creative evil, artist, tinkerer, father of five creative hooligans.
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