Bondo as a mold shell?

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CommanderTano

New Member
I've been researching mold making techniques, reading articles, watching videos, and researching products for months in preparation for a new project of mine. I'd like to make an Ahsoka Tano costume and the lekku/montrals have proven quite difficult. My plan as of right now is sculpting the headpiece out of clay, making a mold from the sculpt, and then casting the piece out of self-skinning urethane expanding foam (most likely something from the Smooth-On FlexFoam iT! series).

My main question for this thread is about making the rigid shell part of my mold. I will most likely be using a brush on silicone mold, but that will not be nearly strong enough to keep its shape when I cast the expanding foam into it. I have seen plenty of videos that suggest using plaster or resin for the shell, but I have not worked with either of those materials in the past, as I am still pretty new at making props. I used Bondo to make a Red Hood helmet last year and I feel somewhat comfortable with it after that project. Would I be able to use Bondo as the hard shell for my mold? If not, what is easiest for a beginner to use for a mold shell?
 

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PropReplicator2

Sr Member
Bondo would crack or chip easily when used in Mass quantities. I would say, since your a beginner, you would have better luck using plaster bandages for your shell. The bandages come in a package with plaster on them. You soak the strips in hot water then just lay them on the silicone! Then your shell is done!

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pitchforknumb

Sr Member
When I use bondo for mold shells I typically make a rondo mixture with fiberglass resin first to get a surface coat, especially if my mold wall is water clay. I'll do one layer or rondo mixture, and then follow that up with 2-3 layers of fiberglass. Really don't need a lot of that since it's just holding silicone in place. Finished up the second half of a mold I made this way this morning.
 

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Leigh

Sr Member
RPF PREMIUM MEMBER
I use fibreglass with polyester resin, 2-3 overlapping layers of fibreglass soaked in resin applied by hand (gloves on) & with a brush.

I've also used what we call P40 over here (UK), its basically fibreglass chopped strand pre mixed with non-activated polyester resin used for bridging small holes in car bodywork, its very much the same as your Bondo but with glass strand mixed in rather than chalk dust. You just add the activator, mix it up & apply it.

HTH :)
 

CommanderTano

New Member
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I'm really bad at taking progress pictures, so this is the only one I have on my phone. I used a plastic Iron Man mask, completed the helmet structure with cardboard, and then covered the cardboard with paper mache. Since it was my first time working with bondo, I struggled with getting the layers to be smooth (which is pretty obvious in the picture) and it took tons of sanding and some drywall spackling to fill the gaps to get it as smooth as I could. This picture is from before I got the electric sander out and really went to town on the thing. I wouldn't really recommend this method because it was very difficult to work with the bondo without the proper tools. Also, it results in a pretty heavy helmet. I plan on making a new one sometime soon out of eva foam and maybe some plastidip.
 

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CommanderTano

New Member
At first I was just using paper grit, but then I switched to a sanding tool that my dad has. It still took ages to get it smooth and I was on a time crunch because the convention I was going to was coming up fast, so I really didn't get the chance to get it to where I wanted. It ended up much smoother than the picture that I posted, but it still had some gaps and dents.

- - - Updated - - -

DarkHelmet, I've watched that tutorial before but haven't looked into epoxy enough. I'll check it out again, thanks!
 

mvaefx

New Member
For a beginner i would use plaster bandages-cheapest and easiest for your mother molds.
You could use bondo but you would have to incorporate polyester resin and fiberglass mat to back it as bondo by itself is too brittle.
 
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