Smooth-on Trowelable Plasti-Paste

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plastipastemixThere are times when a mother can be a real pain in the neck (and no, I’m not talking about you, Mom).

I’m talking mother as in “mothershell,” or “mothermold,” i.e., the hard shell that supports the flexible silicone or rubber mold you’ve made of your model. A flexible master mold is an essential; it acts as a safety net in case your moldmaking goes wrong and you lose your model. It also allows you to build a library of basic or oft-used shapes. When you need to

Without a mother, your lovely silicone mold collapses into utter flaccidity. That’s great if you want to manipulate your silicone into different shapes (and you don’t mind burning your fingers with hot wax). But if you’re reproducing a model exactly you want the daggone silicone to stay put.

Normally, I craft my mother molds out of plain old plaster of paris. It’s cheap, relatively quick and the cleanup is easy. But the resulting shell is also heavy and brittle, hard to store and prone to chipping off in exactly the spot you need it to be whole. Smooth-on’s Plasti-Paste promises to alleviate at least 75 percent of those problems, so I ordered a batch and gave it a shot.

Plasti-Paste is a rigid, fiber-enriched urethane plastic that is relatively lightweight. It’s not cheap–a bit more than $50 for what I judge will be enough to make three of my bas-relief pate de verre molds (as a guide, they’re normally around 16×20 inches and perhaps 6-8 inches deep), costing about $17 per shell. (Turns out it made two, so about $25 per shell). A plaster mother of the same size costs me about $3. So for me to buy more of this stuff the mold will have to be very light, very rigid and very easy to get off the silicone model, i.e., worth more than five eight times the price.

So…here goes.


Cardinal rule of mold-making #1: When testing unfamiliar materials, go outside!!!!
Don’t mean to be a noodge (lordee, I love that word), but the practice of casting glass often takes you into the realms of nasty stuff and–even in a “well-ventiliated room” you can run into trouble, especially with mold-making materials. Since I’d just as soon live a rich, full (and long) life, I do the respirator thing, the eye-protector thing and the gloves thing…and for at least the first attempt, work outdoors where there’s a lot of fresh air and the ability to run fast and far if necessary.  I also pack plenty of paper towels, trash bags and nitrile gloves for protection.

It must work, because I’m typing this with all ten fingers.

Cardinal rule of mold-making #2: Get everything ready to go before you start mixing!!!!
Most investments, shells, rubbers, etc., have a limited “pot life,” that is, they’ll start to harden, rubberize, melt, or whatever it is they’re supposed to do in a reasonably short time. If you mix and THEN set up your model, you’ll be wasting time and material, and be tempted to junk up your mold by using semi-dead stuff. Knowing this from very sad experience, I therefore set my silicone+model on a rotating stand, EXACTLY as I would need it when applying goop…BEFORE I mixed up the Plasti-Paste. I arranged my paper towels, protective cloths, addition stirring sticks, gloves, camera and whatnot, put on my gear and measured out my raw materials, mise-en-place style. I use disposable containers and utensils for measuring and mixing–you could spend a lifetime never really getting moldmaking stuff off containers otherwise–and the cardboard paint tubs are fabulous for this purpose (as are old coffee cans).


Smooth-on strongly suggests protecting your silicone mold with additional release (normally you don’t need much of a release at all with silicone, and none if you’re applying plaster of paris), so I smoothed on two coats of mold wax, let it dry, and sprayed two coats of Universal Mold Release on top. You can, optionally, coat the silicone with aluminum foil, then do the release thing, but I crossed my fingers and prayed this was enough.

Plasti-Paste is a two-part urethane, with a sticky, cottony-fibrous Part B that mixes with a syrupy brown Part A in a 3:1 ratio, by volume. It’s a relatively easy mix, especially since you don’t have to worry about de-aerating the stuff, although it’s a bit hard to stir thoroughly in any volume. The instructions said not to waste time applying it like paint but rather dump it over the silicone and start spreading, fast.

I immediately saw what they meant; the stuff quickly turned into a rapidly thickening, very hot and fibrous mass. Quick-like-a-bunny, I dumped it on the mold and started moving it around with tongue depressors (in retrospect, I would sacrifice a spatula for this instead–the tongue depressor was ‘way too small).

It gave me maybe five minutes before it was too stiff to move, but I managed to get it spread fairly evenly even if it wasn’t very pretty. In later applications it slowed down a bit and gave me maybe six, seven minutes, but still, this stuff goes FAST. The instructions give a working time of 8-10 minutes, is optimistic.

Smooth-on says this material sticks to itself so that you can apply multiple layers. That’s necessary, given the short working time and the difficulty of mixing large quantities of the stuff evenly. My mold was about six inches high and 20×22 inches; much larger than that and I’d strongly consider asking a second person to help with the application.

I had planned to incorporate wooden supports on the two long edges of the shell, giving me a very stable mold stand, but it set ‘way too fast for that much messing around.

plastipasteinteriorOnce you’ve gooped and smoothed (and generated a surprising amount of heat), you let it cure for 90 minutes.  Smooth-on says you can heat-cure for additional strength, but it didn’t seem necessary. The shell was thin and rigid. I’d taken care to leave no undercuts or keys in the silicone that might have caught, so the shell lifted right off the silicone.  (whew–no aluminum foil needed)

What I now have is a thin–about a quarter-inch–strong and lightweight support for my flexible silicone mold, one that will let me pour as many waxes as I need. It also serves as a storage container for the silicone. (Silicone isn’t as vulnerable as a latex rubber mold, but you still need to take care of it) Smooth-on has a nicely done slideshow that covers this in even more detail.

I compared this shell with the one I did for another pate de verre portrait, Trevor. His plaster mothershell is about an half-inch thick, weighs 20 pounds and keeps chipping in places. It’s tough to store and has to be protected from moisture and heat. (Six months later), the Plasti-Paste shell has been stored in an outside shed, exposed to kiln heat, easily slid onto and off of a top shelf on tiptoe, and has held up well.

So…pretty cool stuff. Worth the money. I’d still like to find something cheaper (I mean, 25 bucks is a bit pricey for the OUTSIDE of a mold), but I’ll take this stuff.

Oh, by the way: For the sake of completeness, the mold that this shell is covering is also made of Smooth-on products. In this particular case, I used Mold Max Stroke, a flexible, tin-cured silicone.



  1. Pete Angel September 18, 2014 at 1:38 am - Reply

    Cynthia, do you know this stuff is just overpriced body filler? (In Australia we call it auto body bog or just bog). It’s used to fill dents in cars.

    If you go to eBay and search for auto filler you can get a 3 litre can with hardener for $25!

    Pete Angel

    • cynthia September 18, 2014 at 8:06 am - Reply

      No idea, Peter, but if it is (we call it bondo here) I’m definitely on board. I may pick a bit up and give it a try. Thanks!!!!!

  2. cynthia June 9, 2009 at 8:50 pm - Reply

    Hi, Dafne. Had a problem migrating the blog to a new site and a couple of the resource lists disappeared. I’m putting it back now, and you should be able to get to it at these links:

    Glass blogs
    Casting/sculpture resource list


  3. Dafne Martinez June 9, 2009 at 2:22 pm - Reply

    Love your web site! May take your advice on Warm Glass bulletin board about using a blog for site.

    Anyway, I was trying to see the “little resource list” but I keep getting errors.
    Any other way to access?

Comments welcome! (thanks)

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