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I'm referring to a flat sided mould, not intricate detail. It is the reverse (wall) side of a piece of coving.

I'm using "Formula" Fine Casting Plaster. The wood is planed softwood scraps I had lying around.

Wood grain and other imperfections are acceptable on the cast plaster surface as it faces the wall. My main goal is to ensure I can release the plaster easily and re-use the mould.

I've read that plaster doesn't adhere well to wood, but I'd rather be sure. Will it be sufficient to fill any holes and sand it smooth, or would you suggest varnishing or priming the wood? What about a release agent?

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  • I think this is off-topic for DIY. How about crafts.stackexchange.com?
    – jwh20
    Commented Feb 11, 2021 at 11:42
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    It is for a DIY / home improvement project and I'm using plastering and wood material. Seems pretty on topic to me.
    – Tim
    Commented Feb 11, 2021 at 11:44
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    This is utterly on-topic.
    – Ecnerwal
    Commented Feb 11, 2021 at 14:07

2 Answers 2

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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 "house plaster" is (now) uncommon and may tend towards "hire us to come do this arcane art" more than "here's how you do it" advice (possibly including bad "here's how you do it advice," I have not really looked for such, but I've seen similar things on the web in other "arcane" fields...)

Typical release agents include "products sold for the specific purpose", (again, see ceramic suppliers) vaseline, liquid soap (rubbed on & 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.

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  • I'm going to have a go with liquid soap and report back. Actually washing up detergent is the closest I have. Thanks for the tip about leaving it to dry.
    – Tim
    Commented Feb 11, 2021 at 18:33
  • I accepted this because I ended up using washing up liquid and the plaster is safely out the mould. Inspired by your comment about waxed paper I also taped a few bits of masking tape to avoid plaster getting in to any gaps. The surface of the tape is quite waxy, so I think this helped.
    – Tim
    Commented Feb 13, 2021 at 12:43
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My grandfather taught us to use talcum powder for the mold release. We both made latex molds that the original pieces were dusted then put in the compound until set, and then we dusted the mold and filled. With the really intricate roses and angle wings we misted the mold and then dusted, dad tried the silicon spray but it did not work with the originals the spray soaked in so we always used talcum powder after that. This was for making the ornate pieces on Victorian homes that most folks believe to be carved but they are plaster that is glued on and painted.

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