# Using latex caulk in molds



## Shdwdrgn (Dec 23, 2014)

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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## Severn (May 13, 2016)

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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## vette-kid (May 2, 2020)

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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## Severn (May 13, 2016)

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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## Aard D'Vaark (Aug 1, 2019)

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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## Shdwdrgn (Dec 23, 2014)

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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## Aard D'Vaark (Aug 1, 2019)

no, paint of any kind won't work..


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## Severn (May 13, 2016)

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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## vette-kid (May 2, 2020)

Id recommend something made for caring rather than caulking. My guess is the caulk will be a messy endeavor and won't turn out good results.

If you don't want to use plaster, you could try epoxy putty.

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## Stumpy (Mar 19, 2013)

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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## DaveInTheHat (Nov 13, 2011)

Smooth-On has mold release that would work. You could try using PAM spray.


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## CTValleyRR (Jul 26, 2014)

Shdwdrgn said:


> 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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## Shdwdrgn (Dec 23, 2014)

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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## Shdwdrgn (Dec 23, 2014)

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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## vette-kid (May 2, 2020)

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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## RonnieC (Jul 8, 2020)

Try walnut oil, does't spoil or smell like olive oil will


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## Shdwdrgn (Dec 23, 2014)

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.










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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## Lemonhawk (Sep 24, 2013)

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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## vette-kid (May 2, 2020)

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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## DaveInTheHat (Nov 13, 2011)

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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## Aard D'Vaark (Aug 1, 2019)

what are you trying to accomplish with this ?? it can't be lighter weight as caulk pretty well weighs as much as plaster of paris ... and it's doubtful on flexibilty as caulk doesn't have any when it's cured ?? ....
why not an air entrained solution [such as sculptamold] to cut down on weight ??


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## Shdwdrgn (Dec 23, 2014)

@DaveInTheHat -- I've never heard of paper clay? Does it remain flexible? And for what it's worth, most of the latex caulking (and even some of the silicone ones!) I've seen on the shelf is paintable. I've been using the stuff to smooth out some of my foam work and even the acrylic sticks to it just fine.

A quick bit of feedback on the results from yesterday... No new cracks appeared after the first hour or so, and the caulk cured without losing any detail, so overall it's looking promising. And the cracks may end up looking fairly natural after I slap some paint on this. It did stick to the paper backing I was using, but that is holding the cracked bits together. I think I'll just take a knife and cut out the individual rocks, leaving the paper stuck to it. Honestly I think with just a little touch-up to brush in some watered-down caulking into a few of the cracks, these all look like they will be perfectly usable. The result is good enough that I'm going to order another mold or two this evening.

Now to find some appropriate paint colors and try to make some of these look like the local granite...


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## DaveInTheHat (Nov 13, 2011)

Paper clay is basically ground up paper and Elmer's Glue. Some people add a little joint compound to it. You can use just about any kind of paper from toilet tissue to cardboard to make it. I like using egg cartons. If you do a Google search you'll find a bunch of different recipes for it. You can buy it already made up at craft stores or art stores. It usually costs $8-$10. It isn't flexible at all. It's really hard to break and probably strong enough to drive a car over. If you want to do a test take a hand full of toilet tissue and soak it in really hot water until it starts to come apart. Then wring out as much of the water as you can. Mix it with enough Elmer's glue until its the consistency of soft clay. It dries in about 24 hours. There are a bunch of videos on YouTube of people that use it to make all sorts of stuff.


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## vette-kid (May 2, 2020)

Aard D'Vaark said:


> what are you trying to accomplish with this ?? it can't be lighter weight as caulk pretty well weighs as much as plaster of paris ... and it's doubtful on flexibilty as caulk doesn't have any when it's cured ?? ....
> why not an air entrained solution [such as sculptamold] to cut down on weight ??


Yeah, I've been trying to figure that out. He seems pretty intent on making this method work for some reason. There are several products that would be lighter and more flexible than plaster and still paint able, yet would pour and cure easily.

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## DaveInTheHat (Nov 13, 2011)

If you mix Naphtha with silicone caulk it will thin it out enough that you could pour it into the molds.


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## Shdwdrgn (Dec 23, 2014)

Well the thing is I've never used any of those products before, and I'm already using the caulk on the layout anyway. I do believe caulk is lighter than plaster when dry because it's not as dense. Also, most every latex caulk I've every worked with remains flexible after it cures -- just look at what's in your tub. Vette-kid, you seem to have the idea that this method is somehow more complex and difficult to achieve than other methods, and I don't really understand why? Yes I need a release agent to keep it from sticking to the mold, but that was obviously very easy to find and work with. Then I just squirt in some caulking, cover it with a piece of paper, and press it with a putty knife to squeeze out the air pockets and push out the excess. The whole process takes about a minute to fill all the spots on the mold.

Anyway, I started playing with the paints last night. I'm still pretty much a newbie when it comes to landscaping, so this was the first time I've tried painting rocks. I looked up some pictures of Colorado granite and found a lot of reds and grays. I toned down the red with some beige and painted that on first, followed by a wash of dark grey.









It still seemed a little too bright so this morning (the picture above actually has a lot of shadow over it) I added a wash of lamp black (all of these are acrylics). These two are pretty small (you can see the handle of the paint brush in the upper-right which is only 1/4" around) but there's a great amount of detail highlighted by the paint. And the cracks I had in the caulk either filled in completely or were highlighted by the last black wash and added to the detail, so I'm pretty happy with this first attempt. (The lighting on this picture is much better, truer to the actual color.) I'm still seeing some bright reflections from the lights so I might hit these with some dullcoat to see what happens.









I've noticed the caulk is like a sponge. Not only did it completely absorb the spray oil (there was nothing to wipe off and it hasn't affected the paint), but it is also absorbing the wet paint. If I don't wipe off my wash within about a minute, all of the moisture gets sucked in, so I have to be quick with it. In that regard it seems to be very similar to plaster.


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## vette-kid (May 2, 2020)

I got the impression it wasn't working, but it appears I was wrong. Looks like you have a workable method.

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## Shdwdrgn (Dec 23, 2014)

I probably would have needed to figure out the technique for ANY material I used to cast these. I've done metal casting before, but that's a far cry from working with these little things!  And I tend to post a lot of "here's where I'm at now and it's totally failing" type of replies in my threads, only to have a working solution 5 minutes later. The process of posting gives my brain a chance to reset and see where I went wrong, so it's as much for myself as it is for anyone else who might read the thread later and wants to avoid the same pitfalls.


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## Stumpy (Mar 19, 2013)

They look like rocks.


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