If you have some cavities in the surface you could just gently press in some soft Chavant clay to fill them. You might even be able to use a mould wax to smooth out imperfections (as long as it is compatible with platinum silicone if that’s what you are using.
I honestly don't remember what we used but, I think that you should test whatever substance you want to try, against the silicone first. To make sure it doesn't compromise the silicone curing. A soft clay you know to be silicone proven might be it.