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Brush-On Urethanes Vs. Brush-on Latex Molds
The benefits of using latex are numerous. It is easy to use because it is a single-component rubber that is water-based, making clean-up a snap. It is very thin and can usually be brushed on surfaces with very little preparation (no release agents needed). Molds made out of latex are very durable and may store for over 20 years. Latex is a good material if you would like to cast plaster, cement, and epoxy.
There are, however, some drawbacks to using latex as a brush-on material. There is a high shrinkage associated with its cure, so fine details can be lost from the original in the process. Also, the cure time is 12-24 hours, so only 1-2 layers can be built up per day. This means it may take several weeks to make a single mold!
The use of urethane as your mold making material has several advantages. It has a low shrinkage and thus fine detail can be obtained. After applying a thin layer to pick up the detail, thick layers can be pasted on relatively quickly, so a brush-on mold can be completed in one day.
Urethanes, however, are very adhesive and the original pattern should be prepared correctly. Porous surfaces (plaster, wood, etc.) must be sealed and a release agent should be used as well. Urethanes are two-component systems, which must be accurately measured (sometimes with a scale) in the correct proportion and mixed before application.