Smooth-on Trowelable Plasti-Paste


There 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 “mother mold,” the hard shell that supports the flexible silicone or rubber mold you’ve made of your model when casting glass. (Yeah, yeah, I know that in SOME places the mother mold refers to the entire mold structure, but here in the Northwest I’ve been corrected so much I’m giving in and just referring to the rigid support shell.)

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. 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 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 and full 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 expired stuff. So…I got my silicone+model set up on a rotating stand, EXACTLY as I would need it when applying goop.


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.

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). 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 could see what they meant; the stuff quickly whipped into a hot, fibrous, golden mass. 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). The material was stiffening quickly and rather difficult to move, but in the  five or six minutes I had I managed to get it spread fairly evenly. (The instructions give a working time of 8-10 minutes, which is ‘way optimistic)


Smooth-on says this material sticks to itself so that you can apply multiple layers; given the difficulty of mixing large quantities evenly and applying them in the short working time, I’ll most likely do that next time.  I had planned to incorporate wooden supports on the two long edges of my mold, to make a neat little wax-pouring stand, but this stuff was too goopy and fast-setting for that much messing around. I may try it again on the next mold, but this time I’ll construct a four-sided box and set it down on the curing goop. Much faster that way.

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 I probably don’t need to do that. 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.  (whew–no aluminum foil needed)

What I now have is a thin, 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 ran this trial on a new Vignette piece, May, which is about the same size as an earlier PdV, Trevor. Trevor’s got a plaster mother mold that weighs 20 pounds, is hard to store and already chipping in places. So far May’s shell isn’t chipping, it weighs about six ounces (or maybe a bit less) and is light enough to stow on a top shelf.

All in all, this is pretty cool stuff, worth the extra money.

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.

2 thoughts on “Smooth-on Trowelable Plasti-Paste”

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

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