Silicone is not that easy to "thin". I've seen a lot of YouTube videos posted by garage magicians adding almost anything they could get their hands on to "thin out" 100% silicon caulk for casting purposes. Mostly fails. Even the success's are "failures" IMHO. The grad students in our research labs did a fair amount of PDMS casting and it isn't easy. They had to make up the mix, degas with a vacuum pump and then anneal/bake the stuff to actually make the molds. Usually took 24 to 48 hours from start to finish.
I watched this guy's video on basic silicone cut molds for resin casting.
https://www.youtube.com/watch?v=Rw1NzWHwqH8
He has an entire series on various casting techniques. I learned a lot in about 20 minutes, mainly how I am not going there.




