What kind of plaster?


D

Dougget

Member
I was going to stop by Hobby Lobby and pick up a tub of plaster. All I was able to find was Pottery Plaster. Not sure if that is what I need, so I didn't buy it. Did some research and found all kinds of plaster, pottery plaster, hobby plaster, plaster of paris, etc.

What are good options for my layout? I have some rock molds that I would like to use for exposed rock faces on hill sides. I might also scratch coat some scenery surfaces (foam) to hide seams. I guess I really need a general purpose material that won't crack like drywall mud might.

Thanks,
Doug
 
For covering foam I have had good luck with Lightweight Joint Compound. This doesn't seem to shrink and crack like the standard Joint Compound. There is also a Vinyl Spackle that should work well. Both of these I buy at the local hardware store.

Not sure about Rock Molds except that I've read that Hydrocal is prefered by many.
 
Doug, I used 3 types of plaster. Structolite/Gypsolite , Sculptamold, and Hydrocal Lite for rock molds.

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Plaster of Paris is the 'usual stuff' for pouring into moulds and for making hills and covering joints, especially if you use plaster cloth of the type that a person can get comparatively cheaply at medical supply stores...it's what they use for casts. Recently, though, people have begun to look for alternatives, and almost anything you feel like paying for goes. It can be Sculptamold, Hydrocal, spackle, joint compound, drywall mud, even expanding foam crack filler. If you take the time to use it with learning, patience, and care in mind, taking into account its properties, you can get 'bout anything to work across applications.

Pliability, shrinkage, resistance to paints, sandability, adhesion, tearing/fracturing strengths...these are all considerations. For example, in my experience, although not recent, Hyrdrocal is a bear to paint. It is very hard, very tough, very durable, and like a water-shedding plastic when it comes to acrylic paints. Plaster is easily sawed, scored, broken, sanded, painted. But it breaks easily. Many of the wet spreadables I mentioned will shrink, and it is worse with thickness. But, once dry, they'll sand nicely and take paint.

One thing to consider, too, is that all gypsum-based compounds are hydroscopc. They will absorb water when dry, and they will absorb the water from overlayers of more, or adjacent applications of, plasters, and they will soak up paints. Also, when you spray with glue mixtures to fix ground foam applications to simulate grass and bushes, it will absorb that, too. You'll have to get into the habit of pre-wetting the dry stuff when you apply other water-solubles nearby.
 
If you are having trouble with cracking in your plaster, cut it about 20% with fine sand. The problem will go away. We cut hydrocal with silica in glass casting and it works extremely well up to 1600F. Also, buy it at a lumber company or a ceramic supply in 50lb bags. Keep it dry. The milk carton containers from woodland scenics are a seriously expensive way to get the stuff. Joint compound cut with fine silica works as well. Structolite plaster is another reasonable choice.

I would not cut plasters with sand being used in rock molds.
 
Lighter still, and just as effective, are perlite and vermiculite. It is what I use in my own scenery, fine-ground vermiculite mixed in a ratio with plaster of Paris and Portland Cement. I add a pinch of masonry dye powder, too.

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Great modelling, Grampy!
 
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