Arcane masonic knowledge:
Sugar. Sugar retards the curing of concrete and even stops it from fully setting.
You coat the forms, or anything you don’t want cement to adhere to with sugar. A layer as thin as a haze.
Seriously sugared water, almost a syrup, in a spray-bottle for example. Wet entirely. Let dry.
Or look for or make sugar paper. Concrete will not attach to it.
But of course in real life one wouldn’t use carcinogenic PVC at all when god has made cardboard-tubes to be better suited the purpose in every way. Think posters, coffee, oatmeal packaging.
(Somehow related: Mop & Glo, not thinned with water, applied then dried multiple times, makes it easy to remove cement from porous or rough surfaces. Like removing remnants of grout from Mexican talavera mud tiles. You just have to prepare in advance, if in a tiny project it doesn’t save time it still saves shittons of elbow-grease.
Some of the competing brands may work, some don’t. Use Mop & Glo if you’re in the US.)
