Mold Building

I'm going to be needing a lot of rock molds on the diorama I'm building for the little Porter. That means I've been on the look out for good rocks when I'm walking my girls down in the creek. That normally isn't a good place to look for "parent rocks" because the action of the creek polishes the stones into round boulders. Sometimes I get lucky and find a good subject and drag it home.

So what makes a good rock? I'm looking for a rock that has character and looks interesting, but I also want it to look prototypical for a scale version of the area I'll be modeling. I also look for an area where I can use the mold at different angles so my scenery doesn't repeat the same fissures over and over. If you pay attention to geology, you'll see that there are rules to how rocks and cliffs appear. I also want a rock without tight creases or overhangs that could snag the latex or hang onto the plaster cast too tightly.

I call the good rocks "parents" because they will have many offspring after the molding process. I give the parents a good scrubbing with detergent and a power nozzle. I don't want anything coming off and sticking to the latex. Make sure that the parent is completely dry because wet latex thins with water and we don't want that to happen. You also want to buy quality latex. I hate to say it, but don't buy it from model railroading stores because most of them have had their latex on inventory since they first put that Trojan in their wallet. I'm convinced that when a hobby shop goes out of business, that blue bottle of Mountains in Minutes latex just gets dusted off and Walthers sells it to the next guy opening a train store. I buy my latex from high volume craft stores like Michael's. The best I've found is called Mold Builder which is 100% latex.

Before I start describing the process of making the mold, I've got to cover my butt and urge you to check out this Latex Allergy Warning.

I take a cheap disposable brush and just paint the latex onto the parent. Make sure you get into the nooks and crannies and above all else -- keep thin layers of latex. It will dry within a couple hours and then you can apply another coat. If you keep your brush in a zip-lock bag between coats, you can use the same brush throughout the 20 or so coats you'll be applying to the parent. If you put on too thick of a coat of latex, you'll have pockets of wet latex that may never dry. It usually takes me about four days to finish a mold.

On rare occasions, I will strengthen the latex mold by putting on a piece of cheesecloth before the last two coats of latex. I only have done this with molds that I don't want to distort while molding. Since I want variety with rocks, I don't usually bother. When it is time to take the mold off the parent, just start peeling back at a corner and work slowly.


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