Can you use smooth on silicone on hard rubber item?


Well-Known Member
Hi All -

I just got a screen used prop in the mail - Its screen used "stunt" actually - and its a kind of cool knife from an OK movie -
Vague I know, but I want to figure out my question before I say what it is -

Its hard rubber, and I wanted to make a resin copy - Can I mold it with silicone like smooth on 25/30 ? Do I need a special release?? Will the silicone damage the rubber, etc -

Thank you so much!

Use some mold release and youll be fine. Depending on the finish, I would potentially used petroleum jelly (Vaseline) in place of the mold release. Just apply it lightly with a chip brush then use a heat gun and heat up the coated surface to get it nice and even. This will also give a smooth shiny surface to the pulls from the mold. Keep in mind any release material could have potential damaging effects on the original rubber part pending the rubber itself and/or the paint.
It's probably a urethane rubber prop. When it comes to silicone, general rule of thumb is that nothing will stick to silicone BUT silicone. That said, mold release is a good idea. I'd use some 301 spray which is essentially VERY thinned down vaseline in a spray can. Just lightly mist it and that's all you'd need.

Planning on doing a two part mold? Maybe be the best bet.
Read all the tech on the silicone before you use it as well. Some of the Smooth on silicone will not cure on urethane rubber, you'll end up with a gooy mess if you don't seal it off properly.
All good info. Thank you! I'm going to the store tomorrow and I'll do a test run. :). Not sure if the item is cool enough to offer here..... Hahaha. I'll see after the test pull
maybe make a mold of it from alginate first and get a copy from another material (plaster of paris ) and then mold the plaster copy with silicone. Then you dont need to be worried that the silicone might stick to it.
As people have said silicone will only stick to silicone but if its a rubber/foam piece and has any paint already damaged then be weary. I cast a hard rubber piece and despite my precautions alot of paint came off aswell.
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