making fake rocks?

[illuminis]

I was reading into the false rock enclosure, but curiosity has got the better of me. I was wondering can fake rocks or basking spot be made with 100% Poly-expandable foam? I mean sure i know it has to be coated with a non-toxic water-resistant PVA.

 

[Reapage]

I'm currently making on that has a rough frame of wood then shaped it with great stuff sawed off down and sanded it. Covering it with 2 bags of grout and am sealing it.

 

[Keegan]

So I am new to the site and what I am asking is if I use the "Great Stuff expanding foam" what do I use to seal it so it won't hard my beardie Mekoo