Attaching separite pieces to complex silicone model...

Hi everyone!

I have created a  silicone model (see attached pic) but it is in separite pieces and I was wondering if I could get some advice on the best way to attach them. The model is made from Platsil Gel 25.

I have bought clear RTV silicone  to attach the piece to the main trunk at its point of contact but am unsure how to get a smooth silicone transition that conceals the - at present - very visible seam. I have been told I can paste silicone over it with more appropriately coloured platsil gel 25, thickened with tinthix. Does this sound feasible? Im worried there might be a transparency that leaves the join visible through the silicone. I am also worried that the new layer wont be smooth.

On a seperate note - I also have slight colour differation between the pieces. I am planning on painting the model with silicone pigments - is this correctable during this process or will I have to recast?

Any info on both these questions greatly appreciated


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I don't measure it at all, just chuck in enough to thin the silicone down to what I need. It evaporates pretty quickly, so you may find you need to add a bit more to get it to flow.

I imagine the amounts will be pretty similar for the 2 types, although Gel 25 is normally less viscous than Gel 10.

Maybe there are others who do it all a bit more scientifically, but it seems to work OK for me. 

Hope the painting goes well. Do post a pic of the results!

Hi Simon,

Sorry to keep bugging you but you are literally my only responsive point of contact - no pressure! .

I have been applying the patched silicone to my model and some areas have been succesfull.

I have managed to attach the snout and arm. But I am noticing that the very thin bits of the brushed on silicone that merge with the existing silicone aren't curing very well and I have just been able to literally peel off some patching I did yesterday where the seam was a bit unsightly.

Do you have any idea why this might be and how I can avoid it?

Sorry to hear about the problems.

I am assuming that the silicone you are using to patch the seams has not had any lighter fluid added to it? It should be straight mixed silicone, with whatever pigment you selected. It's quite possible that if you have thinned the silicone with lighter fluid, you have ended up with something that is not structurally sound. This is OK for painting, where the stuff just has to stick, but not good for filling seams etc.

I would suggest peeling the new stuff off and starting over. Try using the method I described, of using baby powder (or translucent powder) either on your fingertip or on a flattish spatula, to gently press the filler silicone into shape. The powder stops the sticky silicone from coming away with your finger/spatula, and when cured it can be removed. Of course if you can get it on with a brush, that's good too.

So for thinning silicone that is going to be anything other than a paint layer, the only thing you should be using is deadener (dunno why they call it that). This softens the cured silicone and reduces the viscosity a bit for mould filling etc. But as you have seams to deal with, using softened silicone is not so great, as the strength of it will be less than the surrounding silicone.

BTW have you seen this video?

At about 26:00 onwards he shows how to smooth seams. But note that he is using tin-based silicone.

Hope this helps! Keep at it - you'll get there!

Hi Simon

Thanks for the quick reply!

No the silicone i'm using to attach the pieces and fill in is just A & B plus a little thickener.

I don't need to thin the silicone for anything else other than painting either. 

I attached the end of the trunk and an arm to the main body of the model. The trunk 

is fine (totally solid) but the arm hasn't cured as well so im a little confused as to what could be the issue. Other than that I may have been off with my A & B measurements for the arm batch as i mixed them both seperately. 


OK, so if one batch worked and another didn't, then it is most likely the mixing. The problem is that as quantities get smaller errors become proportionately larger, so a few grams makes no difference in a batch of 100g, but a lot if the batch is 10g. 

The other thing to watch out for is adequate mixing. It is generally a good idea to mix for longer than you think necessary, and make sure you scrape everything off the sides of the container. Don't just mix until the colour looks right - the components need to be thoroughly combined.

It's good news that one mix worked, so you just need to remember what you did right that time! I know it can be frustrating, but try to use the failures as lessons for the future. Sounds like you are getting there, though!

Yes! Definately getting somewhere now - have managed to patch up some nasty seams
and both seperate parts are now solidly attached!

Im thinking it was down to errors in measuring A & B as the amounts I was mixing up were very small - as you said. I have a little shot glass size measuring cup now with increments on the side so its all working fine.

Will post a pic when its painted up - though ill probably beback tonight with another conundrum!

Thanks for your help Simon - really appreciate the guidance,


Hi Simon,

Have managed to solve the majority of the issues listed with relative success.

SO I now have a hole at the bottom of my silicone model. (see attached pic) 

The model is padded on the inside with Foam Flex-it X. 

Ideally I would like there to be a small flat surface that will allow the model to stand upright independently.

What do you think would be the best way to go about this?

Not sure whether I need to pour a little more foam flex on it so its more pronounced and 

then create the flat point of contact in the foam then paint over it with a flesh coloured silicone?

But the silicone wont stick to the foam. I can start from the outside and merge the existing silicone with the new to create flat surface but wondering will the silicone/foam be sturdy enough to allow it to stand up independently?

I was thinking of inserting a small plastic circular pizza tripod (see pic) in the centre as a solid point of contact and then filling in the silicon around it? Do you think thats a realistic possibility?

Hope this is making sense.



Heres another picture of the base so you can get an idea of how I want it to stand.
p.s haven't powdered yet so its still a bit shiny.



I have just had an idea - do you think if I got a desert bowl (half circle) with a circular flat bottom at its centre and poured the coloured silicone in that would provide me with a solid piece to attach to the base that would already have a flat surface incorporated into it? (see pic)


It's been very difficult imagining what your puppet actually looks like, until I saw one of the pics. Quite a monstrous blob! (Still hard to work out actually how big it is, but it looks fairly huge.

So the issue is: getting a decent flat on the base for it to stand on. Did you want to incorporate any fixings into the model to anchor it to the stage floor? If so, you would need something rigid like the plastic pieces you showed in the picture, with one or several nuts attached to it, into which the tie-down(s) would be screwed. I like using T-nuts, because they have prongs and can be embedded into wood or used as feet.

But I'm slightly off-topic, as you asked about making a flexible base in silicone. I like your idea of pre-casting a flat silicone disc in a bowl or similar, then attaching it to the puppet. I don't like the idea of a thin layer of silicone over the cut back foam, as it won't have much strength. If you need solidity, then something rigid seems almost essential, whether it is wood or plastic trivets.

My other thought is to go with the cheap builder's silicone, the stuff that comes in tubes. It's quite thick, and can be spatula'ed into position, will stick to silicone, and if you put this on, then stand the puppet on something like a piece of plastic (like the flat lid of a plastic storage box), it will get a nice flat base to it when peeled off. The stuff I used was glazing silicone, although the acetoxy stuff also seems to work OK. (Try it on a test piece first.) To get the best attachment to the Flexfoam, you need to give it some sort of mechanical key, by making the surface of the foam quite rough so the silicone can grip it.

Hopefully that will give you a few ideas to be going on with. The puppet looks amazing - quite gross!!

Thanks so much Simon - I have opted for just covering the base in several layers of appropriately coloured silicone. I had a good think and the sculpt was never sposed to stand up anyway. It was designed to be suspended - held from its top. I have bought  a football display plinth so the person it's for can display it standing up right if they wish as the base has ended up being pretty much spherical. Its unnoticable there was ever a hole there! Some careful colour mixing and very gentle pushing round of half cured silicone with a glue spatula seemed to do the trick!

Have attached a couple of pics - these are old now - before the base was sealed. Though there is still some paint work to do and the final matting with powder I think I am over the hump.

Thanks for all your help and support on this Simon. 


Wow! It's huge and fairly disgusting (which is a compliment!) I didn't realise it was on that scale. Looks really excellent. I used to do a lot of prosthetics for Television (Casualty in particular) back in the days when we used gelatine. Silicone is so much more versatile and friendlier.

Found a tip recently in a video about silicone mask-makers. They use a fine Lyra mesh on the inside of their moulds, that gets cast into the silicone when they pour. It helps to prevent big pieces from tearing.

Anyway, hope the shoot goes well. Yuck factor 10!

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