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A:
You want
try to have enough rubber on hand for making your mold all at
one time. You should purchase what you need plus an
additional unit just in case you come up short for some reason
(for example, your mold box leaks and you need to add more).
Reason:
Some rubbers are better than others, but generally speaking,
it is difficult to get new liquid rubber to bond to rubber
that has fully cured.
Result:
Layers will separate (delaminate).
If using
silicone rubber . . .
Liquid
silicone that is applied to already cured silicone will have a
good chance of adhering. The bond may not last however and
you may experience delaminating between layers.
If you
add new rubber to recently poured rubber that is still
“tacky”, the layers will bond.
If the
mold rubber goes beyond the “tacky” stage and is allowed to
fully cure, there is still a good chance that newly applied
rubber will stick. Make sure that the cured silicone is clean
(wipe down with solvent) before mixing and applying new
silicone.
If using
urethane rubber . . .
Liquid
urethane rubber does not stick to rubber that has fully
cured.
If you
add new rubber to recently poured rubber that is still
“tacky”, the layers will bond. If the mold rubber goes beyond
the “tacky” stage and is allowed to fully cure, applying new
rubber and getting it to stick is very difficult.
You can
try it, just make sure that the cured rubber is clean (wipe
down with solvent) before mixing and applying new rubber.
You may
get enough of a bond to allow you to use the mold for one or
two castings before the layers delaminate.
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Smooth-On