Thanks guys
I'm going to be doing some more work on it tomorrow probably. I ran out of screen - something else I have to go get more of.
I did the waterproof test of the rockwall yesterday, and that turned out good. I didn't really explain the painting process fully so here it goes:
First I used acrylic craft paint (diluted) to cover the entire face of the rockwall to paint it a flat grey colour.
Next I enhanced the texture and 3d effect by applying a darker grey colour to the shadowed areas, again making sure that the paint got into all of the nooks and crannies.
Once that all dried, I carefully went over the entire thing again to make sure there were no bare spots - this is VERY important for the next steps.
After everything was completely painted, it was time for the oil based spray paint. Most if not all spray paint eats styrofoam, but the acrylic paint provided some protection against that.
Spraying at different angles, from more than twice the suggested distance written on the cans, I used both Sand and Camouflage Green to add more colour variation. This was also the first coat of waterproofing.
Once dried, I added three thin clear coats from a distance. By then the rockwall was almost completely waterproof. I did one more coat at the suggested distance on the can for good measure.
Like I said before, once that was dry, I installed it into the frame with silicone, and then applied a generous bead of silicone all around the perimeter. Since that wasn't all that sightly, I took a left over can of black spray bomb and painted the silicone edges, and in between each "rock" so everything matched up. This also made the shadowed parts look more indented.
To test for water proofing, I poured a little water on a few spots on the wall, making sure that nothing soaked in. I didn't find any spots, but if I did I would have applied another clear top coat.