My limited experience with mold builder latex product sold at Michaels.
http://www.michaels.com/art/online/displayProductPage?productNum=gc0514
This latex product’s strength is its flexibility. When what you are casting is an odd shape, the flexibility helps to get the part you are casting out of the mold. Also, you don’t need to mix parts, as it’s ready to use out of the container. It smells like ammonia, even when used outside.
I tried casting a number of things from wheels to other railroad parts. The main difficulty in doing this is sagging. Thus, I’ve really only had good success with rock molds, although the product claims it can be used for other objects (I’d be interested in hearing some successes and how it was done).
As to the advantage of its relatively low cost, that is offset by the large number of brush on layers that are usually required and the drying times needed. It may, in fact, be more cost effective to purchase a ready made commercial rock mold. It is recommended that medical gauze be used to fortify the mold.
After about a dozen latex brushings, I soaked gauze in the mold builder and slapped it upside the rock. After it dried, I added about a half dozen more brushings (the amount of time spent doing this, I could have delivered pizzas and earned enough to buy a commercial mold). The whole process took 3 days and the mold endured bright sunshine and driving rain.
When I peeled off the mold, I had slaking in the rock, which resulted in layers of rock peeling off and embedding in the latex. Some pieces came out OK but others pierced the mold. In our area, there’s mostly sedimentary rock. I’d advise casting igneous or metamorphic rock instead to prevent slaking.
For mold release, I used canola oil and for casting material I used vinyle concrete patching concrete mixed with some red and brown latex for coloring.
The process worked but is messy and time consuming. I thi