Need help with rock molds Please

Discussion in 'FAQs' started by Conrail, Sep 22, 2005.

  1. Conrail

    Conrail Member

    I bought some foam shale cliff variations that I will need to make molds of. I have 4 total but one was on back order and is quite large. The two smaller ones pictured are a firm /stiff foam while the larger one is a soft flexible foam, I have no idea what type of foam the one on backorder is made from. I have never made any type of mold in my life but this is the only way I can afford to to do my layout as these pieces and the one on back order set me back $100.00 alone. I am planning on just finding or fashioning a tray with sufficent depth and size laying the foam rock structure in the bottom face up and applying several thin coats of latex, reinforcing a few layers with gauze. My question is wether or not I should prepare the surface of the foam rock in some way to help with releasing the mold? The surfaces are pretty detailed in some parts with nifty undercuts and such. What recommendations might you make before I ruin something or waste alot of material? Thank you for your advice... I'd be lost without it. [​IMG]
  2. ezdays

    ezdays Out AZ way

    You say, "very large", how large are these? Are the surfaces hard? If not, they need to be treated so that the will not be absorbent. I've made a few molds, but they were small and of brickwork for walls and the originals were all hard styreme plastic. I did use Pam to spray the original just to make it easier to release the latex mold, and I did as you plan, put on a few coats of latex, then imbed a sheet of cloth before adding a few more layers. I also sprayed the mold when I made the casting. I'm not sure it was necessary on such a small casting, but I did anyway. One I cast in an epoxy, the others I used plaster and they all came out reasonably well.

    I'm sure there are others here that have more experience with larger rock castings, so maybe we can both learn from what they've done.
  3. Conrail

    Conrail Member

    I didn't want to make it sound as if though these are HUGE but they are fairly larger than any of the WS rock molds the largest is 27½"L x 9½"H x 1¾" thick, the smallest is 13"w x 9"h x 1½" thick. They are all made of foam so I imagine they probably would be aborbent to some extent or another. Maybe there is a sprayable clear coating of some type that is foam safe? :confused:
  4. TrainClown

    TrainClown Member

    Don is right. If your molds are too porous then the latex is going to stick too well and you will have a hard time getting it off. If it was me, then this is how I would approach this project. I would decide that the rubber molds are going to make multiple castings and therefore, there is no need to keep the current texture and paint job on these original "slugs". So I would paint them with latex paint. Use an eggshell type latex paint so the surface will be smooth and resist moisture penetration. I would give them two or three coats to make sure the pieces were well sealed. Thinner coats would be better to help retain detail. Once the paint has dried thoroughly (I'd say at least a couple of days), then I would give them a spray of PAM to make them even more moisture resistant. Then I would use a clean brush to work the PAM in and wipe off the excess from the details (you don't want PAM pooling up and ruining the detail). Then I would proceed with the latex application. Adding cloth is an excellent idea to add strength and you should add extra pieces around the edges for reinforcement. If you have shellaq this would work very well as a sealer too. Don't worry about the undercuts. The rubber is forgiving and will allow you to remove the mold around these. Be very careful applying the first coat. Use a small brush with stiff bristles and try very hard to remove all the air bubbles.

    I made a display for my fish tank and I used an old tin foil rock forming technique I saw somewhere. You might want to check this out.

    TrainClown [​IMG]
  5. Conrail

    Conrail Member

    Great ideas TrainClown. Thank You. :thumb:
  6. Russ Bellinis

    Russ Bellinis Active Member

    Check out this site for how to use rock molds For his latex molds, he suggests rubbing vaseline into the mold so it just feels greasy, but the vaseline dieappears, no residue. He then prays the mold with white laquer. The laquer paint over the vaseline makes a release agent. Now you can fill the mold with whatever you wish to use to make your rocks out of.
  7. XavierJ123

    XavierJ123 Member

    I am reminded of the instructions that you get with most cleaning products. They instruct you to try it on a small corner of "whatever" to make sure it works. I think this is good advice considering the cost of your product. On another note, I have found it most economical to make molds on real rock: one is a brush on method and the other is a poured on method. On the latter, you have to make a box to pour into. This is not very practical with large rocks. I have made eight molds, brushing liquid latex, some 12", some 6" that I have used over and over. Plaster-of-paris is poured into my molds and then laid on hardware cloth mountains. It drys real quick--depending on how long the Plaster-of-paris has already set---normally three minutes or so. It takes two hands to laid my 12" mold so keep in mind what you will be doing. You might need a helper to handle a really big mold. Good luck.
  8. hminky

    hminky Member

    Try sealing the foam with Kilz acrylic primer. When I do ceiling tile rocks I use it to seal the porous ceiling tile material and it works. The tile material can be repeatedly wetted and doesn't go to mush. It should seal the foam.


    Just a thought
  9. Conrail

    Conrail Member

    I ended up trying some latex construction primer and sealing furthermore with a few coats of clear laquer. My latex should be in Saturday so I will update on how the project turns out. If it's a failure then I have a big stack of old ceiling tiles for no purpose other than being converted to rocks.
  10. scoobyandy

    scoobyandy New Member


    Have any of you guys got any idea where i can purchase a rock mold from as want to make my own rocks etc but am not up to making own mold yet as well.
    Can anyone help me here?
  11. ezdays

    ezdays Out AZ way

    Don't know about where you are, but here in the US I was able to find them at my LHS on a Woodland Scenic's rack. I have yet to use them though since I enjoy making different rock formations just using joint compound and a putty knife.
  12. Conrail

    Conrail Member

    I think my first mold turned out well. It's hard to tell if all or most of the details came out as good as I had hoped. I just poured the hydrocal this evening so I will know for sure of the quality in the morning. I was disapointed in the ammount of hydrocal it took to make one of my rock casts. I imagine it will take quite a bit to make as many casts as I will need. I have to search around and see if I can find it cheaper or just go with plaster.

    I am currently working on a much larger mold about 2.5' long and 10" tall which I had planned on making many casts with for my layout, but I can't imagine trying to use hydrocal at it's retail cost for as many casts as I will need even though it seems to work much nicer than plaster.

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