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Silicone thinners; what is a suitable compound to thin silicone caulk? Thank you. Answered

Preferably non-toxic and inert, to make the silicone caulk/gel more viscous, to allow it to flow into a mould more readily.


Do you mean LESS viscous ?

I've used a few % of silicone oil to reduce the viscosity, but it does make the rubber softer.

Better still, get a proper two-pack rubber.

Hi steveastrouk

Sorry, my misunderstanding of viscous was "runny" or "liquid", now I know it means the opposite, so you are right; what I want to do is make this translucent silicone gel more runny or free-flowing AND softer. Can you recommend an outlet/website for silicone oil?



3 years ago

Others have mentioned (elsewhere) to try "odorless" mineral spirits... all I can say "NOPE"... it's a white gooey stuff, but definitely not the mineral spirits I remember. I read somewhere it's vegetable based.

What DID WORK (for me)... is charcoal lighter fluid... Kingsford Odorless... works like a charm!

I needed my sealant to remain transparent, this did the trick!

It doesn't have an ingredients list... but I figured it was similar to lighter fluid, which others have stated works... slow burning, minimal residue... only in a charcoal lighter fluid, probably even more safe since you cook over its remains.

From my understanding, one of the main ingredients for lighter fluid is naphtha... So yeah it will def work... Used it to make resin molds for figurines I've made, works like a charm...

Also if u want some color, acrylic paint works great, good for mold making so you know each layer you add is covering the previous layer

And I just noticed this post is damn old! XD

Good job figuring that out. I'm spraying silicon over greased bottle with an air brush to make gaskets for my wetsuits. Saves me $30-60 per gasket. I'll prob do an instructable.

I have tried a few things to thin (Make more liquid so it flows) common Automotive RTV. What I find that works for me is CRC's Mass Flow Sensor cleaner.

Put more RTV than what you think you will need (its cheap so waste a bit) in a plastic cup and then spray a little CRC in and mix, add more and mix till you get the flow ratio to what you want. Then let it flow around what you need it to do and wait for it to set up. It will take a bit longer at room temp, a lot longer at 40F.

No smoking or open flames of course, have to say it as Common Sense is a bit thin these days. Tide Pods will clean a lot of stuff, but not your teeth. ;)

I have done some experimenting with this, and I claim there are two tricks required for casting a large volume of silicone caulk. 

The first trick is to find some way to introduce small amounts of moisture (water) into the whole volume uncured silicone, because some small amount of water is needed as part of the reaction which cures the silicone.   If there are regions completely devoid of water, these will not cure, resulting in a volume that is cured on the outside, but uncured and squishy on the inside. 

One way introduce small amounts of water into the entire volume of your mix, is to mix in a drop or two of acrylic paint.  I have also read rumor of small amounts of glycerine (glycerol and water) used for this.  One advantage of using a few drops of acrylic paint is  that this provides a visual indicator for when you have the paint and caulk completely mixed.

The second trick, for the thinner, I have read of people using naptha, or xylene, and I have actually tried naptha, and it seemed to work, but of course it smelled terrible, like gasoline. 

I am guessing that part of the magic with solvents like those, is that a lot of the solvent will just evaporate completely, and not become a permanent part of the cast.

great info and i know this is old but just found partsclearer works grest but like u mentioned xylene is the main ingredient. thanks for this.

Hello all, I know that this post was done four years ago, but I was hoping that you might let me know if the silicone oil worked.

If it did, did you notice if the final casting was safe on skin? I am looking into making a similar project for making silicone masks and other applications, so having this info would ROCK!

Please let me know if you have the time.


3 years ago

use petrol "gasoline" (fuel use in motor cars etc.) only problem is smell of petrol. I used this trick and it work.

Where are you in the world Brendan ?

Co. Down, N.Ireland UK
I've seen suggestions for methylated spirits, and also citrus oil. I'm hoping for something non-toxic, touchable and inert


PM me, and I can send you a small tin from work to see whether it works.

Two-pack isnt' that expensive though. I buy mine from Alex Tiranti in Reading.


On this website forum topic, a member posted very detailed information and advice about using silicone for molds and suggests different additives based on the type of silicone you are using.