How to make Latex Rock Molds Wayne Perrier O Scale West 2008 - - PDF document

1

O Scale West 2008 California Central Coast Modular On30 1

How to make Latex Rock Molds

Wayne Perrier

O Scale West 2008 California Central Coast Modular On30 2

Why ? Q: With such commercial variety available, why make your own? A1: You have a lot of time to kill A2: You like making things from scratch A3: You want your scenery to be unique A4: You appreciate the variety of Mother Nature A5: You need / want something you just can’t buy A6: All of the above

2

O Scale West 2008 California Central Coast Modular On30 3

Prepare your Toolkit

• Spray bottle filled with water
• “Dust-off” canister (for cleaning prototype master)
• 1 inch (disposable) brush
• plastic container (for cleaning brushes)
• cheesecloth roll (buy at any hardware store)
• scissors (for cutting cheesecloth)
• paper towels
• liquid latex (woodland scenics works well)
• digital camera (always take pictures of the rock

area)

O Scale West 2008 California Central Coast Modular On30 4

Typical Prototype Rock Outcropping

• Lots of fissures and individual rocks visible
• Significant depth to formation

3

O Scale West 2008 California Central Coast Modular On30 5

Unsuitable Rock

• Stone is relatively smooth, few fissures, not much detail
• Stone doesn’t “scale” well

O Scale West 2008 California Central Coast Modular On30 6

Suitable Rock

• Stone has good fissure detail; is quite rough and textured
• Scales well

4

O Scale West 2008 California Central Coast Modular On30 7

Suitable Rock

• Stone has good fissure detail; is quite rough and textured
• Scales well.
• I made 6 molds off of this one rock

O Scale West 2008 California Central Coast Modular On30 8

Example: Original Rock Face

• Choose a clear, sunny, dry day.
• Use Dust-Off to clean the rock free of debris and dirt
• Lightly spray rock with water to provide a good base for the latex rubber

5

O Scale West 2008 California Central Coast Modular On30 9

First Detail Coat – wet

• Use paintbrush to work latex into all details including fissures
• Apply latex immediately after water spray
• Don’t worry about thickness of coat – goal is to pickup all details

O Scale West 2008 California Central Coast Modular On30 10

First Coat – “dry”

• Let first coat dry at least 24 hours
• Want no pockets of uncured latex

6

O Scale West 2008 California Central Coast Modular On30 11

Second Coat – wet

• On second day, spray with Dust-Off to clear off debris and dust – this could gum

up your paintbrush and liquid latex still in the jar

• Apply second coat, heavier this time than first coat
• Goal is to give mold some body. The color should be more opaque this time

with little rock showing through. O Scale West 2008 California Central Coast Modular On30 12

Second Coat – dry

• Again, let 2nd coat dry a full day. If there are any uncured pockets (in thicker

areas, normally where fissures are), let dry further

• Note the color of the mold now.
• If some rock still shows through (thin areas), a 3rd or 4th coat may be required.

7

O Scale West 2008 California Central Coast Modular On30 13

Third Coat with Cheesecloth – wet

• The cheesecloth coat provides strength to the mold
• Cut squares of cheesecloth large enough to cover ½ of mold surface area or more
• Apply thick coat of latex to entire mold area in same manner as previous coats
• Lay cheesecloth over this wet coat and “work it in” using the paintbrush. It may be

necessary to apply more latex to the outside of the cheesecloth. O Scale West 2008 California Central Coast Modular On30 14

Third Coat with Cheesecloth – dry

• Let the last coat dry 48 hours if possible
• Extreme care must be taken in removing the mold.
• Start at one end and gently lift the edges around that end.
• Work your way across the mold, never letting the mold stretch (and possibly rip).

8

O Scale West 2008 California Central Coast Modular On30 15

Resulting Mold – underside detail

• This was a fairly large mold. These can be difficult to work with
• After mold release, carefully pick out all rock debris from the latex, taking care to

not stretch any area and possibly rip detail away.

• Hit mold with Dust-Off for final cleanup
• Congratulations, mold is now ready to use

O Scale West 2008 California Central Coast Modular On30 16

Other Possibilities

“Glove Mold”

• Allows 360 degree casting of an entire object
• Useful in “standalone” cases, like a boulder in a stream
• Useful for low-weight situations, like modular railroading

9

O Scale West 2008 California Central Coast Modular On30 17

Original Stone and Latex “Glove”

• Note: Latex glove has no cheesecloth. This is because it

needs to stretch for removal.

O Scale West 2008 California Central Coast Modular On30 18

Glove Being Filled with Hydrocal

• Pour “soupy” Hydrocal into glove and use a paintbrush to work it into all the detail
• areas. Fill about 15% of glove. Quickly place paintbrush into water and rinse.
• Swirl Hydrocal around glove, trying to coat all sides evenly, until it stiffens. Repeat

with a second or third coat, after first waiting 24 hours, to buildup a 1/8” thick shell.

• Let dry at least 24 hours

10

O Scale West 2008 California Central Coast Modular On30 19

Original and Cast Stone

• After Hydrocal is dry for at least 24 hours, fill with “Great Stuff” expanding foam

insulation

• Wait at least 24 hours before peeling off the glove. Do not trim Great Stuff foam for

at least 72 hours. This allows the foam time to fully cure.

• If you peel the glove before 72 hours, the boulder could implode