I carved away at the branch feature to get the general shape. Now I am going to coat it with the Drylok. I'm also going to have to stiffen it with some kind of reinforcement; I might use some of this thick anodized aluminum bonsai wire that I have. The Great Stuff foam is really flimsy and the branch shape just bends with any weight at all.
The expanded GS foam has a lot of air pockets in it and I bet that epiphytes would like the surface a lot. I'd like to get a can of the black GS for ponds and try some of it unfinished for growing orchids and ferns.