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is there any way to harden floral foam? Answered

I am making some molds and stuff from floral foam, and I wan't to know if there is a way to harden it and make it heavy.  Some of the things I'm making will have to take handling and painting without chipping, breaking, or crushing.  I use floral foam because it's easy to use, so I can make models in the first place.  I'm not looking for another medium, but how to change it once the model is made.  Also, I cannot use anything expensive or that must be bought online.  Preferrably something like paper-mache, but that will not stick out from small crevices in the model. 


What about using the foam shape to do a "lost foam" casting? I don't know if floral foam burns out as well as other types, but if so you could end up with a plaster mold in the shape of your foam that could receive many kinds of materials.

Good idea, I might try that at some point, but I was looking for a way to solidify the floral foam itself, for weight and cost reasons, the model is about the size of a regular computer tower, so plaster or other solid-core molds would require a LOT of material and be very heavy. If I need a small model, however, this looks like a good choice.

I just found this video wherein the author coats expanded polystyrene with polyurethane resin to give it a hard shell. If the resin is compatible with the floral foam this could work for you very well:

If the link doesn't work, search for:
"XRobots - How To plastic coat foam & expanded polystyrene prop and costume pieces" on youtube.

Maybe use expanding resin foam as a casting material? I'm totally shooting in the dark but that would certainly be very light and not as heavy as you may be anticipating with "casting". A urethane foam would be tough and light, but you may have to play around to get it right. Maybe do some tests starting with small blocks of floral foam with different varieties of sample features and detail. Plaster them, burn them, then play with the urethane foam casting process until you get the result you want. This is so easy from the armchair. I fear I could send you down a twisty garden path if you attribute too much to what I'm writing, so with that caveat, I do encourage you to see if others out there have cast things using a lightweight foam. For that matter, have you considered making a block of urethane foam then carving it instead of the floral foam? It is much tougher. Of course working with it would be completely different. Also with UF you must protect it from UV as it will break down fast in direct sunlight like many plastics.

You could try a varnish or resin of some kind. There are so many products available (in craft shops and hardware stores), and since they aren't marketed for this kind of use its hard to say what would work the best... I know they sell them in really small containers (1 cup size), and they're pretty cheap. Just test it on some of the floral foam to see how it reacts. If that doesn't work, you might have to get something that you can spray on, like a hard drying spray on adhesive that won't absorb as much into the floral foam.

I hope that helps.

I'd try soaking it in 100% silicone diluted with spirits (to make it absorbable). I have no idea if this would work, but if so, the foam would soak it up, and then it may take a couple days or so for the silicone to totally set. Its something I am looking into myself. (post is a year old, but still may be a relevant reply for others stumbling here as I am ;^)

For options that you can find at your local hardware store there are a couple of options I have used with success.

joint compound works very well. It can be sanded perfectly smooth. You will gain strength by doing this but if dropped it still might crack. joint compound will paint nicely though.

Another option is Durhams water putty. it will not go on as nicely or paint as well as joint compound but is stronger and is waterproof.

If you want to get more complicated but still stay within what you can buy from the hardware store try sealing the object with two layers of wood glue and then coating it in fiberglass resin. Resin is extremely hard and sands and paints well.

Another option might be to uses Bondo. Since floral foam stands up to spray paint it might stand up to Bondo as well. I have not tested it though. (you could also coat it in glue and then cover it in bondo) Bondo is softer than resin but sands and paints well.

You might try using the urethane or closed cell foam they use for insulation in houses. This is easily shaped but more robust.

Try painting the model with polyester resin as used for Fiber glass repairs.

I'd try soaking it in liquid wax.