Step 5: Paint
After I was satisfied with the way the rock faces look, I applied layer of light grey latex paint. The foam soaked up the paint so it took several coats before it looked right. After the first coat was dry, I installed the mountain over the fireplace to ensure the train would make it through the tunnel.
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.