Preventing and fixing pits in an ultracal mould

[fallimar]

Hey all you experienced mould-making folks,
I made my first attempt at a mould last night. I was very careful to get a good detail coat on before burlapping and got rid of any air bubbles I saw, but I've got some rather nasty pitting in my mould. I'm wondering if it was caused by the original detail layer being far too runny and causing the water to separate out in a few places, beading on the surface and expanding when the mould cured? Either that, or does it simply look like air bubbles I missed?
Any advice to avoid this in my next mould would be awesome.

See here:


Secondly, I still want to use the mould, so I'd like some advice on fixing the pitting.
I've heard that gypsum-based wood or stone fillers are a good fix, or that you can mix a tiny batch of ultracal and use that, though I'm worried that I won't be able to get either smooth enough to look anywhere near as good as the rest of the surface of the mould. The bits that came out well came out really, really well, it's beautifully smooth and shiny and the wrinkle detail is awesome. I've also seen epoxy putty used.
Any ideas for products and smoothing? I'm casting in latex so I assume a gypsum product would be a good plan so there aren't spots that don't leech out the water. Non-patchy latex curing good. For smoothing, I was considering using some of the original clay form the sculpt that's already the right smooth texture and using it as a sort of small stamp/mould positive. Would that work at all? Sort of like remoulding the mould in small pieces.

Thanks, I'd love to have this be as little of a problem as possible!

 

[Mr Mold Maker]

To stop the Ultracal from beading up, I use Kryolan Dulling Spray.

I've never had pitting like that happen, but whenever I have cracks or air bubbles I fill it in with a tiny, tiny, bit of superglue or epoxy, and fill the rest in with Ultracal.
I also like to think that the epoxy or superglue will hold the mould together just a little bit better than plain Ultracal. Like I said though, Just a little bit of epoxy or superglue in the hole, and when dried, finished with Ultracal.

 

[fallimar]

Thanks. Yeah, I really wasn't expecting weird pitting like that - it's so strange, it was in the clay as well! Big pits in the sculpt. I sprayed it with crystal clear before moulding, maybe that caused more water beading than usual and the heating process caused it to blow out... or it's just air bubbles I didn't catch? Just wish I knew what went wrong!
I think I'll try just filling the pits with ultracal and pressing it with smoothed, sealed clay. The mould itself is rock solid, so no problems with structural integrity thankfully, just these weird holes.

Still, if anyone has a better plan, I'd love to know!

 

[WrenW]

Ultracal tends not to like to bond to itself well once it's dry. Dampen the pits in your mold with a wet sponge. Then mix up some ultracal and water into a thickish paste and put it into a plastic sandwich bag, cut the corner off the bag and use it like a frosting piping bag to squeeze the plaster into the holes. Once it starts to set up a little, use a sponge to smooth out the filling. You can also add a little texture to it at this point.

Best of luck! Hope everything turns out!

 

[Philly]

Ultracal tends not to like to bond to itself well once it's dry. Dampen the pits in your mold with a wet sponge. Then mix up some ultracal and water into a thickish paste and put it into a plastic sandwich bag, cut the corner off the bag and use it like a frosting piping bag to squeeze the plaster into the holes. Once it starts to set up a little, use a sponge to smooth out the filling. You can also add a little texture to it at this point.

Best of luck! Hope everything turns out!

Great first post!

 

[WrenW]

Great first post!

Aw, shucks! Thanks!

 

[fallimar]

Ultracal tends not to like to bond to itself well once it's dry. Dampen the pits in your mold with a wet sponge. Then mix up some ultracal and water into a thickish paste and put it into a plastic sandwich bag, cut the corner off the bag and use it like a frosting piping bag to squeeze the plaster into the holes. Once it starts to set up a little, use a sponge to smooth out the filling. You can also add a little texture to it at this point.

Best of luck! Hope everything turns out!

Thanks! I'll give it a whirl. It's all a bit nerve-wracking first time, the piping bag idea is great. Good thing I'm trained as a chef.

Great first post!

Agreed! :thumbsup

 

[cunningham]

I had one bubble about the size of a zit and I filled it in with epoxy putty, the one sold on tv (Magic Putty I think). It stuck well - like it was part of the mold and almost as hard. That might work for you too, but your bubble is quite a bit larger though. I am curious as to how that happened.

 

[fallimar]

I had one bubble about the size of a zit and I filled it in with epoxy putty, the one sold on tv (Magic Putty I think). It stuck well - like it was part of the mold and almost as hard. That might work for you too, but your bubble is quite a bit larger though. I am curious as to how that happened.

You and me both! I have no idea how the pits happened. I guess I'll just have to make some more moulds and see what happens. Time to get sculpting again I guess!

I ended up repairing my mould by wetting it and just filling with more ultracal. Smoothed it with gloved fingers, it turned out pretty well - it's not perfect but considering the size of the pits I think I did pretty well. The casts are starting to look decent too, yay

 

[cunningham]

Cool, glad you fixed it! So ultracal will stick to cures ultracal, I didn't know that. Can't wait to see the casts.

 

[TripdWyr]

You and me both! I have no idea how the pits happened.

How did you measure and mix your ultracal? Did you sift it into the water, or measure it out and dump it in? Did you mix it by hand, or with a mixer of some kind (drill with a mixing but for example). Did you do it in a clean bucket? Just trying to think of potential failure points here. :/

I recall one of my first molds got this kind of pitting due to unmixed ultracal powder pockets getting washed out when I was cleaning it out. That's when I kinda dumped the powder ratio in and then mixed by hand for not long enough and not even paying too much attention to lumps (Oh, so naive!). Kinda like trying to make chocolate milk and dumping the powder into the milk, you get those little bubbles of powder surrounded by material. I'm thinking you would have caught those lumps though with brushing it on...

Probably not your issue, but just pointing out when something similar happened to me. Let us know if you figure it out!

 

[fallimar]

How did you measure and mix your ultracal? Did you sift it into the water, or measure it out and dump it in? Did you mix it by hand, or with a mixer of some kind (drill with a mixing but for example). Did you do it in a clean bucket? Just trying to think of potential failure points here. :/

I recall one of my first molds got this kind of pitting due to unmixed ultracal powder pockets getting washed out when I was cleaning it out. That's when I kinda dumped the powder ratio in and then mixed by hand for not long enough and not even paying too much attention to lumps (Oh, so naive!). Kinda like trying to make chocolate milk and dumping the powder into the milk, you get those little bubbles of powder surrounded by material. I'm thinking you would have caught those lumps though with brushing it on...

Probably not your issue, but just pointing out when something similar happened to me. Let us know if you figure it out!

Interesting thought. I was pretty thorough, I sifted the ultracal into the water and mixed by hand, but slowly and without churning bubbles in as much as possible. I think my issue was having it too thin to start with and having it bead on the surface instead of glob on a nice detail layer. I'll sort it with the next mould hopefully!