It can work pretty well but you have to coat what your molding with some Mold Ease, Pam, Crisco, something to add a buffer in between what your molding and the silicone cause that **** is sticky. I did a liberal coat of Smooth On's Sonite Wax to my predator trip mine. Then I used a butter knife to spread it over the mine as evenly and I could. You want to keep it about a 1/4 inch thick. Maybe do a couple layers on something like a Bio. then support it with something like expanding foam so you don't get a wonky shaped whatever when you go to pour your cast. Skip the coating and you got a silicone incased prop. I have a predator blade incased it silicone to prove it. I pulled 4 out of the one I made be for a thin area tore. Ahhh experiments.
There's a great way to do this but it takes cost up a bit. Let's start by saying caulk type silicone sticks to everything unless you thin it like some ppl do with mineral spirits, naptha or lighter fluid and they all cause it to shrink and mess up your properties. What I'd do for a cheap(ish) mold is buy a sample kit of a good molding silicone like reboud or even dragonskin works awesome for this. If using a sulfer based clay or any latex, try a tin based silicone like maybe mold max. ( a bit off subject but I only buy from this company now and only from Denver since I had an awesome experience with them taking care of me after a UPS issue and they're a subset of smooth-on anyway http://www.reynoldsam.com/Silicone-Rubber-an/c2/index.html ) Ok back on track, you do 1 to 4 beauty coats with the good stuff and let it thoroughly cure then do a 1/4 inch coat at a time of caulk every day after until you reach your desired thickness. For a bio, using this method, I'd go dragonskin 10 (very stretchy) for the beauty coat and thicken it with thivex to build the wall around the edges, especially the flares and chin. I'd then go cheap stuff for everything that doesn't need to stretch and doesn't directly contact the resin. This way an extra $40 might make 4 or so molds much stronger and give them a lot longer usable life. I'm experimenting with some other options and will likely do a tut one day. The part I posted here definately works tho.
you really dont need to do all that prep for removing your sculpt from silicone. silicone only sticks to silicone. i have used the caulking and a higher grade silicone and all i have ever done is spray the sculpt with krylon. the sculpt always comes out very cleanly. one time i forgot to spray and it came out fine.
the platinums don't often even stick to themselves if the previous coat is fully cured and the tins stick well to other dry silicones (dry meaning non leaching) but the caulking can be unpredictable due to the adhesive qualities. It mostly sticks to silicones, metal, porcelain, and glass. A cheap mold release is made of rubbing alcohol and soap then brushed on in a few thin layers. Agreed on the clay though, no issue there with normal caulking. I should mention though that if you thin it down with something that melts clay and you happen to be molding something clay, there will likely be issues besides the shrinking and oily silicone while it leaches. It might melt some detail out of the sculpt. That plus chemical resistance are some of the reasons a good beauty coat in good stuff is ideal then follow it with the super cheap stuff. Also Walmart has it by the paints for half the price of Home Depot. I hope I don't sound like I'm knocking the previous post. It just got me thinking about some more usefull details.