Do I need a mist watered down coat when using silk paint with added primer on newly plastered walls?

You should only ever use a contract matt on fresh plater before applying your top coats, the matt can be watered down slightly.

Use mist coat of matt emulsion or acrylic primer before apply silk emulsion to any newly plastered surfaces.

Use paint for new plaster for.....new plaster. Dilute with water to the point its easily absorbed, once obliterated and primed you can apply your choice of emusion but it may require diluting too for the first coat

http://v-wilkinsonandson.co.uk/askapainter-pva-new-plaster-painting my blog about the misguided trend to apply PVA to new plaster