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I attempted to make a rubber negative of a bisque fired ceramic mask. I took the mask, coated it with about 5 layers of shellac and then sprayed universal mold release over that. I then pained Smooth-on's Brush-On 50 Urethane over that, and poured a mother mold over that. The urethane stuck horribly to the mask. I talked to the reps at Smooth On this morning and they told me the shellac was the culprit. They said the Universal Mold release causes the shellac to break down. They suggested I use something called sonite wax. Rather than order another product and wait for delivery, I was hoping that there might be some other ideas that might work. Car wax possibly? Any suggestions?

Barb

Shellac is a unique substance that can react in interesting ways. Starting with a completely clean/new mask model I would spray it with several coats of a good quality lacquer (just the ordinary spray can version from the hardware store), give it adequate time to fully dry and then repeat your mold making process with a good coating of universal release and the urethane.

It's interesting the Smooth-On rep didn't mention their Superseal product which is a sealer for porous surfaces http://www.smooth-on.com/Release-Agents ... index.html It can be washed off later thereby restoring the original surface of the model. I've used it with success.

Unfortunately, I don't have the option of starting with a new mask mold. This is a fund raiser for our local art museum - they provide the masks - and artists can do whatever they want. I thought it might be a good learning curve into casting... If I hadn't put the shellac on it to begin with, I could have used the Superseal product... live and learn!

Barb

Shellac and lacquer aren't the same thing, unfortunately. Like glasstech, I use lacquer. I spray on one coat of lacquer, let it dry, then spray a second coat and let that dry. Sonite is a good product, butchers wax will also work. (And I, too, use SuperSeal or OneStep whenever I want to preserve the original, but it's a bit expensive for most of my models)

HOWEVER...urethane and silicone are two entirely different products. Silicone won't stick to anything but itself after it cures. Urethane, in my experience, needs more than universal mold release for porous-ish objects. I rub Sonite or butcher's wax into the lacquered model, then spray one coat of UMR, use a soft brush to work it into crevices and undercuts. I let that dry a bit and then spray a second UMR coat that doesn't get brushed. THEN I pour/brush my urethane. I also subtract about 5-10 Shore-A points (i.e., choose a softer, more flexible product) for urethane than the equivalent silicone product. It seems to help with removal and reuse.

First question: Does the mask have any undercuts that could have trapped the urethane? (just asking) If it does, you'll need to fill those in with clay or wax.

Since you don't have the "new mask" option...first of all, you're in the land of sculptors (or at least the state of sculptors) and if you can get anywhere near Loveland head straight for any foundry supply store. If you can't find that, google "pattern supply." Ask for "butchers wax" or "high-temp paste wax" or just possibly Sonite. If you tell them you're trying to seal over bisque for urethane they'll know what you're talking about. If you don't have something like that in the vicinity, go to an autoparts store and ask for high-temp paste wax. They also have silicone releases, so if you're out of UR or want to try something else, they'll have it. Follow their recommendations.

Clean off the model as best you can, sand if necessary, and work in a good coat of the paste wax. Let it dry, buff off any excess, and apply another coat the same way. THEN spray on the release rather thickly and brush it around to ensure you've got even coating. Apply a second coat, let it dry. NOW try your urethane.

Thanks Cynthia and Glasstech.

Loveland is about 5 miles from my house - I should have thought of that! Thanks!

I'm wondering what product you use for a spray on urethane? What I bought from Smooth-On is very thick - worked fine to brush, but if I'm ever going to do this again with any details, it would be great to know! The basic mask has no undercuts, so I should be set. Thanks much to both of you.

Barb

I spray lacquer onto clay models for urethane molds, as I said, then paste wax, then release. If I'm doing urethane-to-urethane (for example, building a positive mold from a negative model), I don't spray on a sealer at all. The mold is already sealed. I just use the wax, then an appropriate release. UMR is ok on urethane with the wax coat, in my experience...but I tend to be pretty cautious about sealers in moldmaking. I've got a couple of molds permanently bonded to their models/molds, so I make extra sure.

I've also used their wax-based releases (Ease Release) and they do pretty well.

Thanks Cynthia for the clarification. I misread your post - and was thinking that you were actually spraying on coats of urethane. I see now that you were talking about the mold release.

Thanks so much!

Barb