Re: Questions about foam models as safe vivarium decorations
Yeah, I believe you can definitely seal foam adequately, to ensure animal welfare as far as toxicity goes. This I mean in terms of the foam itself, plus the sealant, plus whatever tinting you need to accomplish. I like 100% epoxy solids products. No VOCs, less layers to apply so less concerns with delam / bad adhesion, and you get a quick cure. Acrylic paints I trust, atop the epoxy, not as a substitute. Also the mineral dry tints (though the color options are very limited).
You never heated foam with a heat gun or torch? It's nasty. Avoiding actual flame, the fumes are still awful, and the heated foam surface is converted into something very hard and, if you didn't clean up the surface while it was soft and unheated, scratchy or pokey. There are weird spikes and prickles and stuff, I mean. You can break or grind those off before sealing, but that just adds another step. (The heat-hardened foam is no longer easy to cut.) So I don't much heat foam any more. I mostly just carve it with a serrated blade. And I much prefer EPS to XPS or sprays - I don't like the voids, or the static-cling crumbs / beads. For achieving certain textures heating foam can be very effective, but then to retain that texture you need to apply a more fluid (but still reliably effective...) sealant that preserves the underlying texture - a brushable epoxy resin, or an epoxy paint, for example.
I'm more afraid of residual toxicity with heated foam, I will say that. So I think it requires even more care with the sealing. It's doable, usually I add another layer though. Soft foam I do 2 layers of epoxy, heated I usually go 3. Overkill? Maybe. Probably. Peace of mind? Hell yeah. A little more time, a little more money - so what? Peace of mind for the health of my charges is priceless.
Epoxy sticks just fine to foam that had been heated. That's not the issue. Epoxy sticks to just about anything silicone will stick to. There are some tricks and techniques to working with it. They are well worth learning.
I've never used a hot wire or been in the presence of one in action. I imagine you'd still get the fumes. Probably not the sharp protrusions. I've watched plenty of videos, those tools just seem too slow and there's that toxic smoke thing. Meh - gimme the steak knife, any day.
Hope that helps.