# Plaster of Paris rocks from W.S. Rock Molds



## piperman (Jan 17, 2015)

I just picked up my Woodlands Scenics Rock molds and have a few questions when using with Plaster of Paris.
Do I have to first spray the molds with wet water?
W.S. Says to wait 40 minutes before removing the rocks from the mold when using Hydrocal. Can you remove P of P quicker?
What do you do to clean the molds between casting and before you store for some time?

Any tips on attaching P of P rocks to P of P mountains?

Regards from Cloverdale BC


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## thedoc (Oct 15, 2015)

piperman said:


> I just picked up my Woodlands Scenics Rock molds and have a few questions when using with Plaster of Paris.
> Do I have to first spray the molds with wet water?
> W.S. Says to wait 40 minutes before removing the rocks from the mold when using Hydrocal. Can you remove P of P quicker?
> What do you do to clean the molds between casting and before you store for some time?
> ...


Hydrocal is not the same as Plaster of Paris, Hydrocal is a much harder and stronger material. To make it easier to remove the plaster cast from the mold you could use a "parting compound" that can be in a powder form. I have also heard of using diluted soap for this. On the metal molds for cast lead soldiers I used to use soot from a candle. I would just hold the mold over the candle till it was black with soot, I don't know if the candle flame might be too hot for the rubber molds. 

To attach the rock molds to a mountain of the same material, just use more P of P as a mortar, just make sure both surfaces are clean.


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## ED-RRR (Jun 4, 2015)

*Hydrocal..*



thedoc said:


> Hydrocal is not the same as Plaster of Paris, Hydrocal is a much harder and stronger material. To make it easier to remove the plaster cast from the mold you could use a "parting compound" that can be in a powder form. I have also heard of using diluted soap for this. On the metal molds for cast lead soldiers I used to use soot from a candle. I would just hold the mold over the candle till it was black with soot, I don't know if the candle flame might be too hot for the rubber molds.
> To attach the rock molds to a mountain of the same material, just use more P of P as a mortar, just make sure both surfaces are clean.


I totally agree with [thedoc], because that is what I "Only" use..
Hydrocal:
- used by sculptures..
- allows for a strong thin wall construction..
- most important it allows for adding extra detail (carving)..
......


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## Chet (Aug 15, 2014)

I also prefer hydrocal. As mentioned, it is harder and does not have the tendency to leave bubbles in the castings as I have seen with plaster. I picked up a couple of 50 pound bags at a building supply center years ago and all of my hardshell and castings are made from it. 

I really never wet the old before pouring the hydrocal in. Hydorcal is a bit different to work with as depending on the starting thickness, the working time is a lot shorter than plaster as it will set up very fast compared to plaster. These castings were colored the cheap and easy way. I had a bottle of grungy dirty thinner on the work bench and just brushed the casting with the old thinner.


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## Lehigh74 (Sep 25, 2015)

I use wet water as a mold release…a few drops of dish detergent to a cup of water in a spray bottle. I usually wait several hours (sometimes overnight) before removing the mold from the rock casting. Sometimes the casting will break if I don’t wait long enough. To reduce breakage, I peel the mold away from the casting instead of removing the casting from the mold. To clean the mold, I use an old toothbrush and dry brush the remnant hydrocal bits from the mold. Then run it under water and wet brush. I’ve never attached the hydrocal rocks to a Plaster of Paris base, but I’m sure the doctor is right. When I attach the rocks to rigid foam, I use both hot melt glue and construction adhesive together. Another method is to lay down a base of Sculptamold and press the rock into the Scultamold while it is still wet…makes for a nice outcropping.


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## tkruger (Jan 18, 2009)

I used diluted soap in a spray bottle to coat the molds. Since I had 6 different molds I would pour a set and wait a day to take them out. I have several things going on at once for the train layout so I just move from project to project. At the time if I remember right I was painting also so something was always in the drying stage.


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## traction fan (Oct 5, 2014)

*Plaster of Paris*



piperman said:


> I just picked up my Woodlands Scenics Rock molds and have a few questions when using with Plaster of Paris.
> Do I have to first spray the molds with wet water?
> W.S. Says to wait 40 minutes before removing the rocks from the mold when using Hydrocal. Can you remove P of P quicker?
> What do you do to clean the molds between casting and before you store for some time?
> ...


piperman;

I have used plaster of Paris for rock castings. Also Hydrocal, though, in my opinion, hydrocal is better for building the hard shell body of the mountain, and p-of-p for rock castings. P-of-p accepts paint better, because it is softer and more porous.
With either type of plaster, it's a good idea to use a "mold release". This is simply a substance that helps the mold come off, without ruining the details in the rock casting.
A commonly available substance for this is "Pam" the spray vegetable oil sold in super markets. I have used it with success. Just spray the inside of the mold with a thin layer of Pam, then pour in the plaster. 
As for mounting the castings to the plaster mountainside, there are two basic methods.
The first is "wet mounting". When the plaster in the mold has started to harden into a gel, instead of a liquid quickly pick up the mold and push it lightly onto a spot of plaster mountain that has just been sprayed with water. Wetting the plaster just before applying the mold, helps keep the mountain's dry plaster from soaking up the water from the plaster in the mold, too quickly. This can prevent the casting from sticking on properly. One advantage of this wet mounting method, is that the still soft casting can be formed around the contour of the mountain. This makes the casting look more like part of the mountain, instead of something just put on as an afterthought. Timing is critical in wet mounting. Plaster(either type, but especially P-of-P) dries and sets up FAST. If you wait too long, the casting won't stick. If you try too early, the plaster will pour out of the mold. Only practice will tell you just how long to wait; but here's a tip. When the P-of-P has been sitting in the mold a few minutes, try flexing the rubber mold so as to stretch the back, plaster surface wider. If you see cracks, it's time. Mount that thing now.
The other method avoids all these timing concerns, at the expense of not being able to form the casting to the mountain surface. Just let the plaster fully harden(30 Min. or more)
in the mold. When you remove the mold,(with help from Pam) you will have a rigid rock casting that will need to be glued to the mountain. The glue can be Elmer's type white glue, or more plaster. If you choose to use plaster, spray both the back of the casting and the spot where it will be mounted, with water. Spread fresh plaster on both surfaces and press the casting into place. You will need to hold it in place for 5Min. or so, until it wont fall off.

Hope that helps you;

Traction Fan

PS. Once the castings have dried overnight, you can paint them this way for a dramatic look.
Mix 1 part acrylic craft paint to ten-15 parts water in a household trigger spray bottle.
Lightly spray just a little paint on the casting. Wait a few minutes, and the color will seep into the plaster unevenly giving it a natural look. If it's too light, you can spray again; but you can't go back if it's too dark. Later you can spray on some India ink, thinned with a lot of alcohol. This will come out as "shadows" and really looks cool.


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## piperman (Jan 17, 2015)

Traction Fan
Thanks for the great info. This is all I need to know.
Piperman


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