Next step was to paint the thing. I decided to use tinted Drylok, since I found that it's been used successfully for aquarium backgrounds. I decided to tint it with a blend of "charcoal" and "terra cotta" cement pigments. I was hoping to get brown but unfortunately the best I could get was sort of a grayish/purple instead.
Here's the background after the first coat of Drylok.
I unfortunately don't have photos of the next few steps. The color after the first coating was too flat and uniform. I mixed up a darker batch of drylok and used a dry brush method to feather and shade the background. This helped a lot but I still thought it artificial and rock-like.
I had hoped to create a realistic bark texture but it was beyond my artistic abilities, so I decided to add some fake vines to try to make it look more organic. For the vines I used some lengths of cotton and vinyl rope in various thickness. First a draped/wrapped them around the background and held them in place with staples or nails. I then covered them with a coat of tinted Drylok but because the Drylok really preserves the underlying texture I thought they still looked too artificial and rope like. So I mixed up another batch of Drylok and added in a bunch of long-fibered Exo Terra "Forest Moss" and then applied that to the vines. I thought this was a huge improvement. Here's the end result:
and some closer views to show more of the details. You can see how well the Drylok preserves the texture of the underlying foam.
Overall, I'm really happy with how it turned out except I'm still not completely satisfied with the color. I think it still looks too purple and not very wood like. I think I'm going to buy some "tan" cement pigment to mix with my current colors to see if I can get it a little browner. I may also try to finish it with some ground moss mixed in epoxy.