What plaster or compound for school volcano model?

[Bolt81]

Hello Experts

Apologies in advance as this is not your regular DIY question... 

I am helping my daughter with a school project making a volcano model, a specific one in Mexico. Instead of just gluing paper, I wanted to use some kind of plaster to apply to the paper base that we've prepared. 

Would someone be able to advise on the type of product I should use? I've looked into plaster of paris, but I'm afraid it might not be suitable for hand application. Also, I've never used it before. I've used cornice cement (once), would that be suitable? Or would it crumble if it is lumpy when dried? I need something with thick consistency that could be applied with hand and that could hold the shape when dried. 

Any other suggestions would be much appreciated!

I've attached an image of the paper base that I was hoping could be rendered with plaster. I could add some vertical sticks for support if needed.

 

[JacobZ]

Hi @Bolt81,

Welcome to the Bunnings Workshop community, it is wonderful to have you with us.

You can use most types of plaster for this application; you would just need to mix them to a thicker consistency to ensure they do not slump while drying. If you do get some minor slumping, you can always wait for it to dry, then apply a second coat over the top.

I'd suggest using either Plaster Of Paris or Casting Plaster and mixing them to a relatively thick, sculptable consistency. If you can pick up a lump of plaster, hold your hand flat, and it doesn't slump off, but it is still workable like a dough, this will work.

I'd also suggest cutting and forming some Saxon 600mm x 3m Chicken Netting over the paper. Plaster alone isn't great at holding its own weight, so some reinforcement from the chicken wire would certainly be worthwhile. You can cut the chicken wire with some aviation snips.

When applying or handling the plaster, make sure to wear some gloves as the lime in plaster can and will burn your skin, which, from experience, is not fun.

Let me know what you think and if you have further questions, please don't hesitate to ask.

Jacob

[Bolt81]

Thanks Jacob for the reply with the plaster sugestions.  I also love the idea of using chicken wire, will definitely use these. 

This is perhaps a silly question. What is the technique to mixing the plasters, especially paster of paris, to a thick consistency? Is it adding more water or less water? 10kg of casting plaster is possibly too much for what I need. 

Thanks

[JacobZ]

Hi @Bolt81,

It's not a silly question at all; all questions are valid. A thicker consistency would mean less water. I couldn't tell you a specific ratio, but I trust you'll be able to work it out. You'll mix it once and then know what you're doing, trust me.

You aren't looking for something solid like a lump of clay, but something more along the lines of toothpaste. You want it to hold its shape when untouched, but it will spread easily when pressure is applied.

Start by putting a portion of the plaster powder into a vessel such as a bucket or mixing bowl, then add a little bit of water, mixing it with something like a UNi-PRO Easy Grip Paint Mixer or even a wooden spoon. Once the water is fully incorporated, add another splash and mix again until all the plaster is the same consistency throughout. Repeat this process until you hit that toothpaste consistency I mentioned above. If you go a bit too far, don't fret, just add a bit more powder.

If you don't mix enough the first time, there's no worries there either, just mix up another batch. If you need to do this, just make sure you clean out your mixing vessel before starting a new mix, as the old plaster can speed up the chemical reaction and cause a fresh batch to set prematurely.

I trust you'll get the feel of it pretty quickly. The worst that can happen is it's a little bit runnier than you'd hoped, in which case you just add a bit more powder.

I'm sure you and your daughter will have a great time; plastering can be good fun.

Good luck, and let me know how you go. 

Jacob