Thanks for all the comments on th set up - I'm getting a big head.
To make the wall etc all I did was glue polystyrine bits together (hot melt glue seems to work best for me), then cut and sand away (80 to 120 grit and a BIG mess) all the bits that didn't look right.
I concentrated on making a few ledges at various hights so Joe could get his temp right. He likes to lay on the ledges under the plastic plants. I also pictured water flow to imagine erosion, shape, etc
Once the shape was right I gave it a quick once over with a heat gun, this gives it a rough finish as the foam beads collapse, depending on the desity of the foam how granular the finish looks.
Having done that I paint it with terracotta sealant (Bondall), this makes it waterproof and as it creates a plastic/rubber membrane it also adds strength to the wall and helps hold it together. This coating can lose some of the fine detail you created ... but no big deal.
Once this is dry I spray on a coat of fake rock (granite style). It looks really fake but it makes a good base to start from. Once that is dry I spray a light coat of black (aim for an 'over spray' effect by holding the can at least 20cm away), angled from the base up. This dulls the fake look and provides a very light shadow effect by concentrating under overhangs etc. Don't over do it, if you do just re-spray the fake rock.
Once all the spraying is done I tend to brush on some highlights and accents. Shades of green for moss and lichen etc, i found green with yellow dry brush* highlights looks really effective.
Again this is all too bright so brief sprays of black dull it down.
Once this is dry the penultimate touch is to dillute black paint in water to create a wash, about 2ml in a disposable cup of water, then pour it over the top a few times.The water runs over the natural flow path and deposits the paint where runnoff would normally deposit dirt on a real rock wall. Make it very dillute and do it a few times if required, too strong and you will have to start again. Makes the rock look weathered and highlights shape.
Lastly, when the water drys off, I clear coat it with a spray, to stop the waterbased paints comming off and help stop Joe rubbing off the colour.
With the water bowl I just didn't paint the waterproof layer where the water would sit, everything above the water line is the same.
long post ... sorry
*dry brush - put color on the brush then wipe it on paper until no color comes off. If you then brush this repeatedly over your rock the colour will only be deposited on the high points, or top of the 'grains', so yellow on green looks like moss, or light brown over dark brown etc
