Ok, got ahold of the smooth Kel Seal brushable caulking to smooth down some of the roughness that the textured caulking gave the board. So in photo order from start to where I am now:
Styrofoam hot glued to it. Next step was smearing brushable caulking to the Styrofoam to smooth out the edges and hide the obvious pebblely surface of the foam as well as the straight razor cuts. Some test smears of paint are the yellowish color. Most caverns when they lit with light are Not grey in color but a wide variety of earth tones, which is the approach my color choice takes.
Kel Seal Smooth brushable caulking and applying to the surface area over the textured caulking I dabbed on 3 shades of paint by simply squeezing the bottles of craft acrylic paint around the board. With a half inch soft brush that I would stick into a cup of water about every other brushing to make the colors blend and bleed, I proceeded to cover the board, going back and filling in white spots that showed after the paint started to dry.
Here is the final shot, not much loose rock in an underground cavern and I only hinted at Stalagmites (ground formations... Stalagmites have to hold tight to the ceiling..) though I could and probably will go and add them at some time. And there you have it.