Pond foam/ right stuff bonds strongly to epoxy.
It's fairly well known that this stuff doesn't bond to aquarium glass. But if anyone is curious I applied a "test blob" to some perfectly smooth and well cured epoxy and it has the strongest bond I have seen between the foam and any other material so far. It appears to be bonded molecularly as I can't scrape it off without cutting through the epoxy. I found this surprizing since it doesn't seem to bond to other plastics I have tried (PVC etc.).
I thought this information may be useful to those of you making DIY backgrounds, fake rockwork, and so forth.