This is a common complaint from experienced users of Smooth-On plastics that we hear often in the winter months. It is a problem associated with urethane chemistry in general. Units of liquid plastic are shipped from the Smooth-On factory (or a distributor) in cold weather and the material gets very cold in transit.
Ingredients in the liquid Part B separate due to being exposed to cold temperatures (weather). Separated Part B combined with the Part A causes the plastic not to cure properly and, once cured, the plastic will exude an oil.
Remedy: It is critical that, before using, Part A and Part B be brought up to room temperature (73 F / 23 C). For a gallon unit that has been exposed to very cold temperatures, this can take 2 - 3 days. For a 5 gallon unit, it can take 3 - 5 days.
After the material has been brought to room temperature, Part A and Part B containers must be shaken thoroughly to re-blend ingredients.
The material is now suitable to use and should perform normally.
What if I have used material from cold containers? Unfortunately, what remains in the Part B container may not be usable. By dispensing separated material, the balance of ingredients in the remaining material may be off. A small test is recommended to make sure the plastic cures as expected.