Hollow stucco castings for outdoor use?

[helix_3]

Here's what I know:

• I can find plenty of videos online showing plaster being used to create hollow, thin-wall castings
• Plaster is not weatherproof
• Stucco can stick to vertical surfaces, has good plasticity, etc- so it seems like it could be brushed into a mold without issue
• Stucco is weather resistant

Considering these points, stucco seems like a good material for outdoor, hollow castings. However, I can't find any evidence of people using stucco this way. I feel like I'm missing something.

Can stucco be used to make hollow outdoor castings? If no, why not?

 

[udog]

Hi
I´m not very familiar with stucco, and have never used it for casting.

I think these would be the questions:
Is it brittle?. Then it will need a reinforcement material behind if casted hollow.
Will it dry in thick sections?. how long would it take to dry for a 1cm section for example?. Some materials will dry in thin sections, but will never do so or take very long a in a thicker section. For example, paint.
Does it have a big contraction rate or not?. Thinking about warping if casted hollow.
Tests in a silicone mold for example would be the way to know. Never heard of anyone casting it either, but who knows.

Google says:
"Traditional stucco is made of lime, sand, and water. Modern stucco is made of Portland cement, sand, and water. Lime is added to increase the permeability and workability of modern stucco. Sometimes additives such as acrylics and glass fibers are added to improve the structural properties of the stucco."

Writing this I´ve just remembered that I have a good friend (a builder) who knows a big deal about stucco, and I watched him doing a small decoration in my house.
But he used plaster and a plaster retardant (similar to UC30) I recall there are different kinds of stucco.
Maybe some of the questions I made above make no sense. Wether it carries plaster or cement and lime this means it will dry in thick sections as there is a reaction going on, and it shouldn´t be that brittle if it has marble powder for example.

Could be worth trying, let us know if you get to something. Sounds interesting and after writing all this I´m starting to think it could work. If hollow, maybe with a reinforcement material like fiberglass or burlap.

 

[NUNMAKER]

Definitely watching this thread, I've been wanting to build a couple large Laputa robots in my garden.

 

[helix_3]

I spent some more time researching, and found professional and amateur solutions:

For professional, there's a video here showing how to make a hollow casting using GFRC, which is very strong cement mix. The Buddy Rhodes site says 0.5-1" wall thickness is recommended. This is great for architectural applications where you need absolute strength and reliability.

For amateur, there's a lot of videos online showing how to make hollow garden spheres/ orbs. This video is pretty good. You take a yoga ball, seal it with plastic wrap, apply fiberglass mesh (aka lath), then smear on a layer of cement. Everyone seems to prefer a slightly different cement product, but they're basically all just Portland cement/ sand mixes. For a mold, I assume you could do a face coat of cement and then back it with lath/ cement.

The amateur method is simple, cheap, and uses easily sourced materials, but I don't know about the material's long-term durability and strength. Cement + lath is a pretty standard construction method, but I don't know if the hollow, thin walls would introduce some kind of problem.

If anyone knows about this type of material's durability, please let me know!

 

[udog]

If anyone knows about this type of material's durability, please let me know!

Yes, that´s a fine material (GFRC and similar). It´s a standard in theme parks for example, and some of them can be sculpted directly as it´s thixotroped material, so durability outdoors should be ok. I´ve seen good stuff made of that, when working on big surfaces and thinner sections I know some use chicken wire fence as a reinforcement for example, and sculpt directly over that. A friend of mine made a huge tree that way. Probably more reliable than stucco.
For casting you might have to get the feel of it to get all the detail registered. It´s a matter of pressing the material into the mold.
Normal concrete/sand mix will need thicker sections by itself, for hollow castings with thin sections you will have to reinforce it.

 

[helix_3]

It´s a standard in theme parks for example, and some of them can be sculpted directly

What would you call this concrete fabrication method? I'd like to learn more about building large-scale items this way, but don't know what to search.

 

[udog]

What would you call this concrete fabrication method? I'd like to learn more about building large-scale items this way, but don't know what to search.

Not sure about the name, if there is. But I´m sure you can find where to start from here for example:

https://www.youtube.com/results?search_query=buddy+rhodes+sculpting+with

https://www.youtube.com/@BuddyRhodesConcreteProducts

Lots of interesting ideas there, and I think taking a look at all the materials they sell will give hints to develop your own.

 

[Gee2]

Just chiming in here for reference; I've lived in the southwest for most of my life, Stucco or STO are the mediums of choice for most if not all our homes outer wall coverings, etc. It works if done correctly but isn't a perfect covering. When/if exposed to the elements it requires some upkeep within 2-5 years due to cracks and separation from the underlayment (this is normally done with an elastomeric paint over an epoxy mix in the cracks themselves). Granted we have some pretty extreme summer temps, so depending on your location mileage may vary. I would advise against its' use as a casting medium as it need something to adhere to and by itself is extremely brittle. STO is a bit more forgiving for temp/weather exposure as it's an elastomeric base but requires some skill for application as the material can "burn" if spread too thin/thick or too fast. Again, not something for a contiguous pour or cast. I would recommend a concrete or fiberglass material for the casting and then maybe a STO exterior for longevity.