The subject line of your post differs from the question you pose in the text, so I'll address both with what I know from first-hand experience:



First, let's clarify that I'm talking about 2 steps in sculpting either a background or "prop" (stump, hollow log, rock outcrop, waterfall, whatever) for an enclosure. Step 1 is creating the actual sculpture, the "body" or "blob". Step 2 is covering the "blob" in something that is both attractive and functional, in terms of environmental resistance (to flowing water, UV light, sharp-clawed little dragons, heavy crawling snakes, adherent or penetrating plant roots, etc).



GREAT STUFF VS STYRO (sculpting the blob)

- GS can be a little hard to control coming out of the can

- GS breaks down with long-term exposure to moisture (goes to goo) so your sealing needs to be perfect

- GS is a sticky nasty mess before it cures; it comes out of the can with some force, and you can end up with some on your face; you'll hope you're wearing eye protection if/when that happens!

- GS is a little expensive if you're "going big"

- nevertheless, it indeed is "great stuff" for many applications, and I always have a few cans around (you can cover the extruder tip with plastic and get more shelf-life out of it)



- both extruded and expanded polystyrene are useful for backbrounds & props

- styro can be had for free (dumpster diving); if you buy it in large sheets, it's cheaper than GS, by volume (cu in or whatever)

- styro does not break down with exposure to moisture

- styro is easy to control - it just sits there until you do something with it!

- extruded (blue or pink, more solid) is easier to carve than expanded (white, puffy)

- styro and GS actually can work really well together; I don't like to use the GS as my adhesive because with expansion it pushes the styro around; any construction adhesive for use with foam board is good for styro adhesive



GROUT VS SILICONE (covering the blob so it's both attractive and environmentally-resistant)

- silicone is a god-awful mess to work with, and pretty hard to control in fine detail

- the acid fumes the silicone throws off while curing are a real health issue; the stuff is just plain nasty to work with

- long-term, the organic material wears off the silicone, leaving you with an ugly black rubbery background

- you can really only get one "look" (color, pattern, texture) from this method; and it doesn't really look all that natural even at its best

- the cost can add up a bit if you use a lot of silicone tubes

- the technique is not that forgiving if you screw up along the way - if you need to go over some coco with more silicone, it's hard to feel confident that you have a water-tight seal between the layers

- if your organic material is not perfectly dry when you stick it to the silicone, it's not going to stick...

- despite the drawbacks I've mentioned, this is a robust technology for very humid environments

- personally, I will never create another habitat-scape using this method!

- the frog guys still like it



- grout & sealer allow incredible diversity of creative expression, in terms of colors, patterns, and textures; you need to see to believe, just how good some "fake rocks" and "fake trees" can be, made with these materials (e.g., see Dendroboard, look up a Finn named HX)

- the materials are ridiculously cheap

- the materials are easy to work with, and you can really improve your sculpting technique with experience and intuition

- the materials are pretty forgiving to work with - if you produce an area or a whole layer you don't like, just slap on another and give it another go!

- you still have to care for your body, but I think these materials are less inherently-dangerous to e.g., lungs and eyes

- choice and application of sealer are critical; acrylic-based products (e.g., Mod-podge, Ace acrylic co-polymer, etc) are fine for xeric situations and "clean" animals, but for humid or wet environments, or really messy animals (turtles, indigos, cobras, whatever) you're much better off using polyester (fiberglass) or epoxy resin

- styro sealed with epoxy has long been a favorite with aquarium enthusiasts, who have also been making their own tanks with plywood, glass, and fiberglass or epoxy for a very long time



hope this helps,

Jimi

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