i use gorilla glue to make backgrounds for poison dart frog tanks. it is 100% waterproof and non toxic when cured. it is really good stuff. i make water features like seeps, little creeks, and ponds with the stuff and i have never had any problems. i have been using it for 5+ years about.
this background is entirely coated in gorilla glue and peat.
i made a creek side type habitat here for some salamanders with gorilla glue and sand.
first, put a generous coating of gorilla glue over the foam and spread it evenly
always be sure to wear gloved when doing this. it is quite messy
spread a nice 1 inch layer of whatever substrate over the gorilla glue and press the substrate into the glue. you will have to repeat the pressing process several times for about 15 minutes as the glue expands and will bubble through the substrate.
only do small sections at a time otherwise the glue gets out of hand. use a shop vac to suck out the excess substrate once the glue has cured. there will always be spots to patch up.
once all the missed spots are patched up, shop vac out all the loose substrate and the end result is a very nice substrate background.
what is nice about the gorilla glue is that it soaks through the substrate quite a bit, but it doesnt soak all the way to the surface. it leaves a very nice natural exterior that plants and moss will grow very well on, but is thick and very strong. it dries almost rock hard. give it a try. i guarrantee you will like it if done right.
-josh