I've been working on this for about a month and it was a great experience. This tank is a 40g breeder, home to a male bearded dragon, Ozzy.
This method can be used for Dry Arid, and Tropical environments. The only difference you'll need to do is change up paint color's and your sealant.
Supplies:
1 can of Great Stuff expandable foam 5$
Polystyrene 0$ (Just Ask Hardware stores, they get tons in their tile shipment)
Red, Orange, and Brown Non-toxic acrylic paint 10$
Mod Podge 8$
2 10lb box's of Non-sanded white grout 10$ each (I ended up only using 1 of the boxes)
And lastly and most importantly,
Time
Total: 43$
I don't have any pictures while I was building, just the end result.
Step 1
I sprayed the GS and let it sit and cure overnight. Once it was cured I started carving/designing it to what I drew out.
Tip #1 Get a basic idea on what you want, but make it random. The more random it is the more "natural" it looks like.
Step 2
I then applied 4 layers of grout on the background.[All of this was done by feel, I didn't use any measurements. (Sorry, for all of you rocket scientists out there.)]
Tip #2 I would recommend coloring each layer a different color. Here's why, the GS (depending on which kind you have) should be yellowish/white. When you use a different color layer you will know where you've been on the background.
Layer #1 It should be thin, so it can soak in the background and get in all the nooks and crannies.
Layer #2 This layer should still be thin, but thicker than Layer #1
Layer #3 Now this is where you start getting away from sloppy stuff. When mixing it should be thicker and a bit harder than Layer #1, and #2.
Layer #4 This last layer should be thick and even harder to mix than #3. Don't make this layer too thick, It shouldn't be so gloppy that is starts to build up on your mixing device. (knife/fork/stick) Layer #3 and #4 should be dry and done in-less than hour. #1 and #2 should take 2-3hours.
Step 3
Now is the fun part, painting! Painting will either be the most simple or hardest thing of the project. Its all opinion based and should be used with your own creativity.
Tip #3 Just go with the flow and let your hands do all the work. Usually the more brains you put into painting the more you won't like your creation and the more time you're going to try to making it perfect.
Step 4
Now is the final part, sealing your background. I used Mod Podge, Non-toxic all surface sealant. I did 5 layers of sealant and
1 thick layer (not watered down) on High Traffic Areas (I didn't use a spray bottle on this one, I just used a paintbrush.) The sealant will make it "shiny" especially your thick layer. A way to eliminate the "shiny-ness" is to use the hard end of a sponge and gently scrub it "off" (It's kinda like when you're nose is shiny because of your natural oils, and you use a tissue to wipe it off.)
Tip #4 A fast way to seal your background is to get a dollar store spray bottle. Put your sealant in the bottle and water it down so that is will flow easily out of the nozzle.
That should be about it. Your end result may turn out a bit like this:
Frontal view
Side View
