That's a trick question because you can use something as nasty and harmful to the environment as diesel oil (and against the law in some jurisdictions) or you can choose a modern concrete form release that is easy to use, biodegradable, has superior performance with fewer bugholes and less staining.
Bad concrete form releases include diesel oil, heating oils, used motor oil, etc. Good, chemically active, soapy types of concrete form release agents are derived from soybeans, flaxseed, trees, hogs, cattle, fish, etc. and produce fewer bug-holes, stains and surface irregularities than bad types. Sure, good concrete form releases will cost more than waste motor oil from your truck, but you will use a lot less, it will last longer on the forms and won't be harmful to the environment.
Here's a YouTube video that illustrates how the selection of concrete form release affects the production of concrete bug holes.
From Paolo Redaelli, "Effects of two releasing agents on the surface of a properly vibrated concrete. Please note the one on the left having a perfect surface after very few seconds while the one on the right keeps forming bubbles of air."
For more information on selecting concrete form releases read last week's blog post, "Selecting The Best Concrete Form Release Agent."
Thanks for your interest in learning more about, What is concrete form release?
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