Put clingfilm over the top, and press it flat with something like a book (or fingers). White spirit has enough water to let the caulk cure even under clingfilm.

Just make sure to remove it outside. The pong is nasty

For a two part mould, you don't want it perfectly smooth anyway, smooth bumps give you a good lock between mould parts ensuring they're lined up.

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My shifting projects

Clingfilm right, something else to take out of the kitchen

I'm lucky in one respect that I'm doing my testing in my gaming shed so it is well ventilated and I then leave the test on the garden table on the way back inside.

Once first side is done to my satisfaction I'll drill small holes for locking the other side in and pour over the top with the rest of my mixture. Hope to then try the casting solution and see how that goes (I even have another food dye ready for that as well).

How did the experiment go?

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My shifting projects

Not as well as I would have liked. I haven't found a suitable releasing agent and have found the second half of the mold sticking.

Did make one mistake and forgot to add the cornflour to one batch, it took ages to dry and then stuck to the test item.

So I'm looking for a suitable release agents to try out.