I have an fiberglass object, that I want to make a mold of. Commonly known is that fiberglass and silicone don't like each other. And when I tried to make the mold, I experienced what I expected got a gooey consistency. I believe this phenomenon is called cure inhibition.

Is there a way that I can make it work? Or is there a different material that would work (soft molding material)?

  • Did you by any chance use air curing silicone from a tube like the stuff for sealing bathtubs? May 13, 2018 at 18:55
  • How close of a match do you need. If you applied a layer of smooth saran wrap to the original and then pour silicone of that, would the variance caused by the thickness of the wrap be too much for your intended use? May 13, 2018 at 21:50
  • @nothingismagic no im used a 2 component silicone from smooth on
    – Frezzley
    May 14, 2018 at 5:46
  • @HenryTaylor the difference in depth wouldn't make a difference however I guess I would be having a hard time getting the details right
    – Frezzley
    May 14, 2018 at 5:47

2 Answers 2


Would a two step process work? Make the first mold off of the fiberglass with latex rubber (or another molding rubber that is not anti-fiberglass) then make a plaster positive from that. So now you have the fiberglass form in plaster, use that for the silicone mold.

  • sounds like an interessting approach - what kind of molding rubber would you suggest for a life size statue that would can keep the form and isn't too expensive?
    – Frezzley
    Aug 9, 2018 at 17:02
  • You should check out the mold making materials manufacturers or retailers for that. sculpturehouse.com and shop.smooth-on.com have a lot of info.
    – rebusB
    Aug 10, 2018 at 15:17

Just use a tin cured silicone.

fiberglass and tin-cured silicone work fine, I use fiberglass over-molds (mother-molds) for silicone molds.

It is only platinum based silicons that have problems, anything that contains even trace amounts of sulfur prevents it from curing, fiberglass included.

  • Interessting, I'll give it a try
    – Frezzley
    Aug 20, 2020 at 18:42

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