The Styrofoam is just what I had lying around from packaging.
I either broke pieces with my hands to roughly the shape/size I wanted or cut them first with a craft knife to slightly larger than needed and then shaped with my fingers/finger nails to rough up the edges.
The first coat of grout was mixed so it was quite runny so it would find its way into all the knooks and crannies.
It takes 24 hours to dry before subsequent coats can be applied which will be slightly thicker (Mcdonalds milkshake thickness)
4 coats are needed in total and then 2 coats of waterproof pva to seal.
I didn't have any plan before I started, just started breaking bits up and letting it develop as I went.
There are a ton of examples if you google diy rock hide.
The best example of a fake rock wall I have found is at Lizard Landscapes -
http://www.lizard-landscapes.com/reptile-terrarium.html
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