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.