Re: Building a large spire out of blocks of eggcrate, what to do about the sides?
I built the setup I linked to with both styrofoam slabs (panel insulation from Home Depot) and the pearls + epoxy method. Most the of the structure was solid styrofoam and the pearls were just the surface (and I could have done without them).
Gluing styrofoam: Silicone will adhere to roughened styrofoam just as well as it does to spray foam. Solid styrofoam is easily glued with epoxy* or gorilla glue or spray foam (FYI: gorilla glue and spray foam are the same material with different expansion ratios). I used West System epoxy and gorilla glue to glue the layers of styrofoam together. The glue is much tougher than the foam - so ideally you want to do your carving mostly before you glue it together and avoid thick layers of glue between pieces. You can still cut and shape it when glued, too - but not as easily. I carved with a steak knife and a box cutter - most any knife or saw will work.
Note: I put an asterisk on the mention of epoxy because there is always a chance, when mixing plastics, that they can react. Always test your epoxy with whatever materials you want to use BEFORE you go all-in on building with it.
Drainage: As for your water-draining concern, you can easily build a drainage channel (or drill drainage holes) into your styro-stack. You could dover the hole or channel with screen to prevent it from clogging. Given their shape, I don't think drainage will be much of a problem for your spire-islands.
Pearls + epoxy: As I mentioned, this was a surface treatment and was not really necessary. I mixed a couple of gallons of styrofoam pearls (you can get them as beanbag filler) with a half-cup of epoxy. The resulting material is a super-light-weight "paste" that you can apply with gloves and paper towels. It took some practice for me to get used to working with it and I think I could have gotten a similar result by carving the styrofoam slabs more. Given the effort and all the time breathing through a ventilator (to avoid epoxy fumes), taking more time to carve the styrofoam would have been a better use of my time, I think.
TL;DR: You can use all the materials you're used to (silicone, peat, netting, spray foam) with styrofoam slabs and they will be much easier to shape into a natural surface shape that will readily take a surface treatment like silicone+peat or even spray-foam + cork pieces.
The best part of this material is that it becomes a very strong pin cushion. It makes attaching bromeliads, clumps of moss, and even small branches very easy since you can use barbecue skewers or stainless steel pins to anchor stuff anywhere on the structure.
Last edited by kimcmich; 04-30-2018 at 02:48 AM.