Concrete-Coated Styrofoam Insulation Board Options & Alternatives

June 5, 2024 3:25 PM

Making a small outdoor fountain out of stacked styrofoam insulation board with the intention of coating it in concrete to give it a rocky texture. Concrete will not stick to styrofoam, so my thought was to glue burlap to the board and then apply concrete to the burlap. I'm a little hesitant to go that route though as the layers of burlap and concrete may obscure the shape of the fountain (Basically going for a striated mesa thing) and it'll end up looking like a big gray turd. Looking for suggestions or techniques that may also be effective for finishing the exterior. Thanks!
posted by Alvy Ampersand to Home & Garden (11 answers total)

My approach would be to paint the styro with a glue, like white glue or acrylic paint or something, or use a styro-safe spray paint (Plutonium is a brand name, and I think they make others for floral and stage sets and stuff) as a barrier, which the concrete will stick to but won't slump and make a turd shape like a textile might. The barrier might allow you to use a conventional wall texture stucco spray as well to help with the rocky texture.

If burlap is the way to go I might want to see it stretched like a canvas and stapled very tightly to the board so there isn't any slumping that way either-- I feel like a glue might peel away under the weight but a stapled stretched canvas or burlap would work better.

Another option is to do some of this in reverse, so to speak--- build a mold for hollow exterior design elements, then use the styrofoam as interior fill. That is probably more involved than you want to get.
posted by blnkfrnk at 3:44 PM on June 5 [1 favorite]

I've used carved foam as base for little sculptures I covered in tile grout and it seemed to stick to it fine, but its possible I forgot a step as I haven't done it in many years. I was using tutorials for people who make sculpted landscapes and terrains for terrariums and pet reptiles.
posted by GoblinHoney at 4:13 PM on June 5 [1 favorite]

Response by poster: Absolutely not married to the burlap or the concrete; I was basically trying to take gaming terrain techniques but replace water-soluble plaster and terrain paste with concrete so it could withstand the elements and give it some heft for the wind. I should definitely try priming/sealing first and maybe try a textured spray paint and see how that works, maybe I wouldn't even need the concrete! Adding grout as texture prepaint is another technique I forgot about, so thanks!
posted by Alvy Ampersand at 6:06 PM on June 5

Rosco FoamCote is made for this. It can take paint on top so I’d guess concrete will stick though you should do a test.

When I built styrofoam scenery we didn’t have the money for the FoamCoat so we used PVA (white aka Elmer’s) glue, and cotton cheesecloth to cover foam before paint. The cheesecloth is very thin so it doesn’t obscure details but it does give a little tooth for the paint to grab on.

I’d try a test: concrete over multiple layers of cheesecloth/white glue as a since that’s going to be cheap and readily available. Then spring for the FoamCote if it’s not sticking well enough. You can thin the glue with a little water to get it to flow better on a paintbrush.
posted by sol at 6:12 PM on June 5 [2 favorites]

Stucco is used for stuff like this too. 3 step stucco - the stucco is held on with chicken wire.
posted by The_Vegetables at 6:42 PM on June 5 [1 favorite]

Forgot to add: to see what it looks like, go to your nearest outdoor mall that has stucco buildings but also has 'traditional' window detailing or other decorative exterior details- that's foam and stucco. Tap it and it sounds kind of hollow.
posted by The_Vegetables at 6:47 PM on June 5

Weldbond is a PVA glue that is specifically supported for this use.
posted by Mitheral at 8:33 PM on June 5 [2 favorites]

Here is a search for stucco mesh. Some good ideas there.
posted by H21 at 9:49 PM on June 5 [2 favorites]

The interaction terms of gravity, water, styrofoam and concrete can be awkward . . .
"The centre-piece was the 30-meter-bak: a U-shaped aquarium for tropical-reef fish. It was a big engineering feat because 30m of glass enclosed 40 tonnes of water and required a substantial steel frame to raise the aquarium to eye-level. We needed to construct a reef: partly to provide some nooks and crannies for the fish and partly to save the punters from having to look through the water at other visitors on the far side of the aquarium. Someone had the bright idea of carving a rough template in 1m x 0.5m blocks of expanded polystyrene, plastering the slabs with concrete and covering them in coral-pink, green, beige and brown camouflage paint. The result looked convincing but when the tank was filled, the reef peeled off the floor in sections, turned turtle and bobbed to the surface. The density of polystyrene is much less than water: a 3cm thick coat of sand-and-cement isn't enough to compensate. The fall-back was to build the reef-wall from lumps of dull red lava, of which we had a heap from an earlier project. That worked out okay."
posted by BobTheScientist at 9:58 PM on June 5

I'm not convinced that concrete won't stick to polystyrene, at least for non-structural applications, but you could try a small test piece to find out. Otherwise, the PVA glue Mitheral suggested sounds like a good way to go. I don't see the need for a binding layer of cloth or anything, but the concrete may need to be thicker than you want to retain its integrity out in the weather. I'd be inclined to use a polymer render rather than concrete, though (which may be the same thing as stucco). It will be more inclined to stay in place on vertical surfaces than concrete.
posted by dg at 12:09 AM on June 6 [2 favorites]

In my youth I built model railroad layouts, using styrofoam blocks roughly hacked into the right shape, then just coated with plaster. Which appeared to stick well enough.
To improve the rockiness of the plaster I mixed a bit of wallpaper glue (the stuff that's probably a hi-tech version of corn or potato starch) into it, making it less gooey.

The other method of building hills and mountains was chicken wire coated with papier-mache, then again coated with the plaster/wall-paper glue concoction.
posted by Stoneshop at 12:10 AM on June 6