How should I prepare a wooden mould for casting plaster of paris?
You may find more (quantity) of plaster casting/release advice looking at ceramics sites on the web - plaster is frequently used for ceramics molds, while &quot;house plaster&quot; is (now) uncommon and may tend towards &quot;hire us to come do this arcane art&quot; more than &quot;here's how you do it&quot; advice (possibly including bad &quot;here's how you do it advice,&quot; I have not really looked for such, but I've seen similar things on the web in other &quot;arcane&quot; fields...)
Typical release agents include &quot;products sold for the specific purpose&quot;, (again, see ceramic suppliers) vaseline, liquid soap (rubbed on &amp; allowed to dry in place, typically several times) cooking spray, vegetable oil. Priming is not needed, the release product will soak into the wood.
Given that you are looking at releasing a flat board, the simplest approach might be to cover it with something like waxed paper or plastic wrap, but any of the release agents mentioned above should work fine. A film product like paper or plastic wrap may release too easily (while you are working) and thus might not be as good as the usual methods, in fact.