This two part rubber used in my moulds above are used in the movie industry to make all kinds of props. It comes in different densities and is easy to use. I use ball bearings to form my mould alignment pins because they are perfect for ease of mould separation. It's way better than Flex-Seal. Less expensive too, due to the volume required for most projects.
I have painted this rubber onto a pattern in several layers then painted fiber glass resin over that. I apply layers of fiber glass for rigidness/support of the rubber layer. When rigid enough and supported with ribs at strategic areas, I remove it from the pattern by splitting the mould between the flanges I made on the fiber glass layer.............Bruce