My Smooth-On plastic castings are partially cured and are exuding an oil. What is the cause?

Article courtesy of Smooth-On, Inc.

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.

Related FAQ: What are the effects of cold weather on urethanes or silicones?

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 already 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.


This FAQ article is offered as a guideline and offers possible solutions to problems encountered during mold making and casting. No warranty is implied and it is up to the end user to determine suitability for any specific application. Always refer to the provided Technical Bulletins (TB) & Safety Data Sheets (SDS) before using any material. A small scale test is suggested to determine suitability of any recommendation before trying on a larger scale for any application.