The second lava room was based on foamcore instead of cardboard so I could avoid having to fill the corrugations with spackling. If I didn't get all the cardboard for free, I'd much rather work with foamcore. I could cut out all the cracks and the foam left a nice smooth side that angled down to the bottom. I cut some of the cracks too deep and ended up gluing a piece of cardboard on the bottom to cover that up. But it still looks fine. For additional depth I glued thin card on top of the foamcore, following the shape of the cracks, but leaving room for the paper to show through. Then I glued the floor tiles on top of the cardboard. So I ended up with four layers.
I used sand to cover the exposed foamcore paper, and a little in the cracks, to simulate the texture of cooled lava. Then it was ready for paint. I used cheap latex as the "primer" mainly to cover the exposed foam and protect it from the spraypaint. Then it was spraypainted black for the basecoat. I started out drybrushing the sand with gray, but the smooth areas didn't have enough texture, so I tried out a new technique (new for me anyway) of using a small sponge square to dab the paint on, leaving a pattern from the holes in the sponge. I'm satified with the effect, you can see a close-up below.
The next step was to paint red "lava" in the cracks, which my daughter helped with, and then some more drybrushing to give a variety of colors.