If you look closely at my nice new rock, you'll see I dropped the $50 piece and glued it back together...
From what I found Gorilla Glue is simiar if not the same as Great Stuff foam and people are using that as faux rock backdrops in their aquariums, painted with Krylon Fusion yet to boot...
GG and GS are polyurethane, and from what I could find chemically inert when fully cured.
Tim