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Discussion Starter #1
Not sure if this has been covered and I haven't found anything online... I have some Woodland Scenics rock molds and I would like to cast my rocks with some colored latex caulk. I did a quick little test today and found that after about 4 hours the caulk starts sticking pretty strongly to the mold so I rinse it out at that point (had to use a toothbrush to get it all clean).

Would some kind of release agent work here, or are the two materials just going to stick together? I was thinking of trying to brush something like baking powder into the mold, but it seems like the caulk would just pull it out of the way as I'm squirting in the initial amount. I dunno, but I'd love to hear any suggestions on making this work? The layout could be transported to shows during the Winter, so extreme temperature changes are anticipated, which is why I wanted to use the caulking for this. Silicone would be acceptable too, except I think it would present even more problems with sticking.
 

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Ok must admit that I kinda like the idea but really have no direct experience. Years ago plaster in plaster molds with yes Vaseline barely worked.

I think Id tried a super fine powder as you said... Although this will be embedded then in top layer of the caulk but maybe it could be buffed out. Or it could just not work.

Another option, something like Pam baking spray.
 

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I would imagine a release agent would work. I've never tried it with silicon caulk, but it work on silicon rubber. I like smooth on universal mold release. I've used it for making silicon rubber molds as well as castings.

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That seems promising. Of course one likes to imagine alternatives since it's surely something like Pam or has wax in it. Etc etc. Well so google suggested '"... carnauba wax" such as is found in paste car wax.
 

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yes, a release agent will help, if you don't have the 'proper' agent handy, a substitute can be used, such as full strength dish soap brushed on and allowed to dry...
again, the final product is time sensitive, and products such as plaster of paris, that 'set' rather quickly, are preferable to use ../ definitely not something as slow as caulk, lol [from experience on my own layout]
 

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Discussion Starter #6
The problem with plaster is it's just so brittle. As I mentioned, this will be transported around so I'm trying to keep everything lightweight and semi-flexible.

I did have one idea that I might try as a release agent... Painting the inside of the mold with acrylic paint. I'm not sure if it would be better to let it dry overnight or only let it set up for 30-60 minutes before applying the caulking though. Drying overnight could mean it sticks too well and doesn't release. Not drying long enough, since both substances are water-based, could mean the paint blends into the caulk and there's nothing left to release.
 

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Id use vegetable oil first. Very light coat ... although I think Pam will stay put better. Just don't need any beads that form, wipe it out a little.
 

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Paste wax for a release agent. I smear it on the jig and clamping cauls when making cutting boards. Even after the glue has dried 24 hrs. it pops loose easily.


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The problem with plaster is it's just so brittle. As I mentioned, this will be transported around so I'm trying to keep everything lightweight and semi-flexible.

I did have one idea that I might try as a release agent... Painting the inside of the mold with acrylic paint. I'm not sure if it would be better to let it dry overnight or only let it set up for 30-60 minutes before applying the caulking though. Drying overnight could mean it sticks too well and doesn't release. Not drying long enough, since both substances are water-based, could mean the paint blends into the caulk and there's nothing left to release.
I hear you on the brittleness of plaster. I would try a casting resin instead of caulk.
 

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Discussion Starter #13
I'm seeing a lot of suggestions for Pam, and it seems like the easiest to try first (we actually have conola oil and olive oil sprays). Since beading could be a problem I might trying brushing it into the mold. ... Although, if olive oil is a possibility, I could always just brush that into the mold instead of trying to spray something?

If this idea doesn't work I might try it with casting resin. Would that also need some sort of release agent, or does the resin not stick to these molds?
 

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Discussion Starter #14
Wow, so the spray oil had an unexpected effect. I dropped some caulk into a couple of the small rock forms, put a piece of paper over it, and used a putty knife as a squeegee across the top of the mold to level it off and knock out the excess caulk (I think this helped to also push out any air bubbles). When I came back a little later and lifted the paper to check for sticking, the whole formation lifted up! So I flipped the mold over and the paper with two formed rocks fell into my hand. This could be excellent news for fast-tracking the formation of a bunch of rocks at once.

Now I have the pieces drying on the table, which will be an overnight process. The real question remaining is whether or not the caulk will retain all of the details as it dries, or if it will slowly sink back into a formless glob. It's been sitting out for over an hour already, so I am hopeful...
 

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Yes, you will still need a mold release. Just buy a spray can, it goes a long way and isn't that expensive, $15-20. Look on the smooth on website, they have a ton of info about different products and what works best. You can also contact then for recommendations specific to your requirements.

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Discussion Starter #17
Well I have some concerns already. I decided to do another casting of ALL the rocks on this mold, and they have been drying for about 3 hours now. I'm seeing cracks forming in both sets of castings I did today. :( I really expected that with the oil coating, it would slow down the drying process enough to prevent this. On the plus side, the surface of the first casting has dried to the touch without any loss of detail, so I expect the rest will also set up nicely.

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This front casting is about 1x3 inches, so it's fairly small, but the detail seems to be retained from the mold. There is already a crack along the right side though. Another issue I've seen is where the caulking has rolled up on itself while I'm squirting it into the mold. It picks up the oil and can't stick back to itself as I smoosh it into the mold, so there's a least a couple of rocks with noticeable lines from this. I think the solution might be to squirt the caulking onto the putty knife, prees it together into a solid lump, THEN scrape it into the oiled mold? It looks like I'll be doing a lot more experimenting to get this technique to work...
 

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I would be tempted to not pack the mold full of caulk, but rather a make a somewhat even think coat on the face to generate better drying.
 

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Sounds like more trouble than it's worth. Not trying to be negative, I'm just not seeing the point. There are products for this that aren't too expensive.

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Maybe instead of using caulk you could use paper clay. It's durable and light weight. Then you wouldn't need to use any mold release. Plus, paint will stick to it. Paint doesn't stick to caulk very well. You can make your own paper clay, so it would be almost free.
 
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