I have a minor problem. I get air bubbles in the plaster that I use for my rock molds. It’s not a big problem, but sometimes I can see round holes in the rock. Is there a way to avoid this?
Gently tap the side of the mold after you pour the plaster into them and the bubbles should rise to the top, which will actually be the back of the rock where they do no harm.
[#ditto]
What he said!
Stir it more slowly, too, EL. I mixed quite a few batches of both Hydrocal and Plaster of Paris for my last layout, and I simply took my time and stirred it like I would sugar into coffee (yeah, I like my coffee sweet). You are not agitating it, as if stirring mud up in water, but more like stirring paint in a tin that had not been used for several weeks.
When you add the plaster to the mold(s), fill it 1/4 full, nudge the side of the mold several times and in several directions, and then fill to half-way, and so on.
I have seen on a DVD that Dave Frary sprays the mold with wet water before adding the plaster. But he never tells why. Do you guys know why? I also wonder if it’s easier to get air bubbles if I mix the plaster too thin?
You can set the bucket on a running table saw, or any shop tool that would provide enough vibration to make the bubbles rise.
Nelson
Spraying the mold with wet water keeps the plaster from sticking to the mold. It’s probably the dishwashing detergent mixed into the water that acts as a mold release more so than the water.
Yes, mixing your plaster to a thinner consistency will allow the bubbles to rise to the top more easily. I use 20 minute casting plaster and mix it to a consistency thinner than pancake or cake batter.
I use the methods described by Cacole and very rarely do I find any bubbles. When a thin mix of hydrocal is poured into the mold tapping on the work surface will bring any bubbles to the top.
Hydrocal, which I use for molds, should be quite loose and pour freely. Plaster of paris or the veneering plasters, should be much thicker in consistancy if used for hardshell. The veneering plasters such as Unical, Diamond and others, have a greater working time before setting.
Use powdered masonry dyes for coloring your plaster if desired.
The wet water will break the surface tension in the mold and allow the plaster to flow freely into every contour and nook and cranny.
There was a recent thread about mold release. Wet water has worked for me on new and clean molds. Some had suggested “Pam” cooking spray. I need to try this on some of the older molds that don’t always release every detail.
Keeping your molds clean is most important for best results. I sometimes have someone washing them immediately while I continue working so as not to let the stripped mold dry out. By turning the mold inside out any particles and crumbs are much easier to remove. Scrubbing w/ a soft brush and an occasional pick returns them to like new ready for more.
Professional molders pull a vacuum on the wet plaster. This is beyond the scope of most model railroaders. All of the above suggestions will help.