Step 1: Design!
Google SketchUp Download
Step 2: Gather Materials and Tools.
Styrofoam ~ Free (This is trash, you can find it for free. It's used to pack all sorts of things. Try going to the back of an electronics store and asking if they've got any you can save from the landfills. I used a broken styrofoam cooler and some odd packing bits.)
Grout mix ~ $8.98 (I got a carton of Sanded grout mix in HEMP color. The sanded leaves a rougher texture than unsanded. The hemp is a kinda dark gray color that already looks like rock. It gives a nice dark base color and could be used without coloration if you want)
Cement Color ~ $4.96 (I used Terra Cotta. Mixed with the Hemp grout it gave a nice red rock color. This is a liquid color mix. You can also use powdered grout color mix. It's about a buck cheaper, but they were out of the color I wanted.)
Great Stuff Expanding Foam Insulator ~ $5.00 (Trust me, this stuff will make things much easier and is well worth it)
Water ~ (To mix the grout)
Water Based Polycrylic Satin Sealer (Go for Satin, not as shiny as gloss and gives a more natural look. You'll need enough for 3 coats. A small can will probably suffice. )
Knives and hand saws (to cut styrofoam and shape your cave)
Hot Glue Gun and glue sticks (works great for gluing pieces together and it's safe. DO NOT USE SUPERGLUE. Superglue, or other such chemical glues will melt/eat through styrofoam and give of toxic fumes)
Bucket (For mixing grout in)
2 Cheap Bristle Paint brushes (One for applying grout, one for sealer)
Step 3: Start Building!
Start with a real rough cut, just laying things out. Don't glue anything down yet. It also helps if you can draw on the floors and wall of the viv to give yourself reference points for shape. If you don't actually want to mark your floor, lay down some newspaper you can draw on)
Get your basic shape together. Don't worry about leaving holes and don't do any detail carving yet.
Step 4: Glue
Step 5: Expanding Foam!
Spray it in to fill in all those little holes and gaps.
Use a little more in places you want it to build up.
Leave it to expand and dry overnight.
Step 6: CARVING!
Step 7: Grout!
Mix your grout. Don't mix it all at once. Leftovers will dry out while you're waiting between coats. Try using about 1/4 of your grout per coat. I don't really have a set measurement for water, but you want it to be kinda thin and soupy. It will brush on easier that way. If you're using coloration, this would be the time to mix it in.
Once you've got your soupy grout, start brushing it on.
Make sure you get in all the cracks and crevasses, including the bottom.
Let it dry overnight between coats.
Apply 3 coats! This will ensure everything is covered and it will be nice and strong.
Step 8: Paint!
Step 9: Seal it.
Use your other brush and apply 3 coats of sealant, once again, making sure to get into every crack and crevasse. Sorry, no pictures here, cause there's not much to see while sealing. Once you've finished, let it air out for a few days, preferably with a fan blowing on it.