Fibreglass over RTV silicone - Oil based clay partition?


Hi folks,

I am about to start making my silicone mould over my cosplay helmet. I am Using Tin Cure RTV silicone and am going to be using Fibreglass matting wetted with polyester resin.

However as the helm will need 3 partitions i will need to lay the mother mold in sections.

While i have done plenty of research i cannot for the life of me find out if this will work:

Basically i have seen online tutorials and vids showing a partition made from card, foil backed paper etc and sprayed\brushed with krylon crystal clear (i assume just a clear acrylic varnish?) or brushed with carnuba wax\vaseline etc.

What i am trying to find out is, can i use oil based non sulpher clay as a partition wall between the edges of the fibreglass sections, with just a coat of wax and spray on mold release?

Or will the clay cause big problems with the polyester resin?

I cannot find any clarifying posts on this, hence why i posted a new topic.

I have 3 weeks to get it all done so any quick help would be great.

many thanks all.
Last edited:


Sr Member
Hello Rick, and welcome to the forum.

Answer: Yes, you can use oil clay for a parting wall. Yes, it will need Vaseline as a parting agent.

Suggestion: Rather than build the entire parting wall in clay, I recommend Foamcore, Masonite, or even cardboard cut to the profile of the mold, and secured in lace with clay. this gives you a clean, flat wall with much less working the clay with tools.


Hi, thanks for that.

That was the clarifying bit i was stuck with, it seemed to be either or.

I will make flat wall with card and use clay as bulk\strength brushed with release agent :)

Thanks a lot.


Hi folks,

Well had a bit of a drama last night with my helmet mold.

I had bout Chavant NSP medium clay to form around the area i wanted to have a seam for the fibreglass mother mold, however i softened the clay in hot water and was able to form it, but it obviously wont stick to the silicone and as a result, getting the fibreglass matt to conform to the flat edge to get a 90' shape was causing headaches (the resin being older though brand new tin, it seemed very thick and caused issues too when brushing on.

My thought i had was to use air drying clay to build up a mother mold edge to put fibreglass onto. I found the NSP clay very hard very quickly once out of the water and also found it didn't want to mix together, like it was made of separate parts that wouldn't form an easy whole, if that makes any sense, basically the clay would tear and where it had torn, was like a natural separation section that didn't like mixing together.

Anyway, i know sulfur clay's will inhibit silicone curing, but i wondered if it was safe to lay ontop of cured silicone for a brief period of a few hours?

If it inst i will use tin foil asma base and put the clay on top of that to create my partition walls.

Cheers folks, need an answer asap, as starting to get close to deadline now :)

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