Help I am stuck !! Decorative Concrete Mold Release Agents
I have been in the concrete business for quite a few years and I have made latex molds and pour concrete ornaments using latex molds. Latex does not like petroleum based products as well as sunlight. Your best bet is a castor oil mixed with methyl hydrate or isopropyl alcohol (1 part castor oil and 8 parts methyl hydrate). Add the mix to a spray bottle and shake it well before each use. Just a light spray with the mixture, remember that too much is bad as it will stop the concrete from setting.
A secondary option that I hear quite often is Pam or vegetable oil. These are ok and will w0rk, but not as good. These will also get sticky and hard on your rubber unless you wash it off after each use.
A third option is a pre-packaged aerosol spray can such as Crete-Lease or Synlube 531 as examples. These work extremely well, but can be expensive in comparison.
As a warning for latex rubber molds any kind of petroleum based oil, such as (motor or mineral) is bad as it will turn your latex into soft liquid mush!
I did come across this nice headless man giving a good demo on mixing and applying the castor oil mixture: Youtube video on applying release agents. He must not like the camera much…
As for fibreglass molds, you can use almost any kind of oil as long as it does not stain your concrete. If the concrete absorbs it, there could be a layer on the outside of your finished concrete which won’t let you apply any finish to the sculpture. I find it just easier to apply the same castor oil mixture.