Fresh silicone rubber caulking, which is frustratingly tacky and difficult to work with,
can be "cut" with mineral spirits (paint thinner) to produce a smooth, flowable liquid
that can easily be brushed onto surfaces.
As with many things, the "devil is in the details", so to speak. There are a number of insights
and caveats that are helpful if one is motivated to try using this approach. These are briefly
outlined here:
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Silicone rubber will tightly adhere to any clean, smooth, non-porous surface.
This is especially the case for this approach because the mineral spirits will, in some
sense, "clean" the surface of residue as the silicone/thinner mix is applied.
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Models made of plaster, which has an open, porous structure, offer a
good substrate when using this approach. It has enough "tooth" for the slurry to adhere
while curing, but is open enough that removal of the finished mold can be accomplish without
tearing. Using water-based clay for models also works. Whether using plaster or clay, allowing the model to air-dry prior to making the mold is important.
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If it hasn't already occurred to you, this silicone/thinner slurry is *amazingly* useful
when it comes to roof repair & patching. I use it a LOT.
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One can impregnate elastic cloth with silicone rubber to produce a very thin, strong and
amazingly stretchable waterproof cloth.
- One can embed elastic cloth directly into a mold at key points where tearing is a potential issue. It adds a lot of tensile strength. Some of the molds which I've made are literally turned inside out during removal (like taking off a sock), which can produce high levels of tension in the mold. Once begun, a "tear" in silicone rubber is deadly, and can quickly ruin a mold. Using fabric can easily remedy the potential for this kind of mishap.