Hopefully I have learned enough from the last two experiments to make a good casting this time.
I set up the wok again with Plasticine walls and a slightly larger saucepan lid and made up a much larger batch of plaster hoping to cast the whole thing in one go.
I poured the plaster into the wok and started swirling it genlty, hoping to make it stick to each part of the surface – impeded somewhat by the thick layer of petroleum jelly. During the next 10 minutes the plaster gradually thickened and began to build a smooth layer over the whole surface area. As I swirled the liquid it behaved like a tidal wave coming around and around in a regular sweep but as it began to set it grew thicker and was in danger of leaving a literal tide mark in an unsuitable way. I set the wok down to rest and watched as the excess plaster solidified in an almost even spare-tyre kind of shape in the centre.
I decided this time not to try to manipulate the surface in any way and I am pleased to say that it is smooth and even, though somewhat thicker towards the centre with a discernable swelling about 2 cms from the pan lid, it looks pink here but it dries white.
I suspect that the outer/upper edges are fairly thin and that I will need to make another pouring.