On Nov 21, 11:56*am, "charlie"
wrote:

snip

new silicone won't stick to old silicone. it's all a physical bond.

Not so. Silicones will have no trouble bonding to clean old
silicones,and the new to old bond of clean material is indeed chemical
and equal to virgin material. I suggest that you actually do this
yourself. Try it inexpensively by using small tubes of sealant
commonly used for automotive gaskets.
There has been some grumbling about the performance of Silicone II, so
perhaps this is related to problems you may have experienced.
People using expensive silicone mold making compounds usually save $$
by recycling chunks of material removed from the molds into fresh
mixed batches for new molds. A slice of the cured resultant compound
shows no delineation at all between old and new. HTH

Joe