See the dark holes? Each and every one must be sealed.
The mold rubber will seep into any tiny gap, so the entire surface is inspected. Any gap between the blocks is sealed with plasticine. That saves wasted (and very expensive rubber) and makes a stronger mold.
Here's the sculpture after all the holes have been filled.
A thick coat of shellac fills any holes I missed and seals and prepares the surface for mold making.