render or bonding plaster to smooth off reveals priop to drylining...

Bonding plaster is a plaster undercoat that also sticks well to almost everything (tools in particular;-). You should be able to get it smooth, although not smooth as finish coat as it contains some larger pieces (which would also mean it wouldn't work in a thin layer).

However, it isn't waterproof. It will sock up water, and prolonged or repeated exposure causes it to fall apart. If you want something which will survive in the presence of damp, then you should use sand and cement with a waterproofer. You could also add a small amount of exterior/waterproof PVA into the mix.

I don't have a completely clear picture of what you are doing, so I'm not sure if this is helpful or not.

In message , Andrew Gabriel writes

I read it that the OP wants to smooth off the internal window reveals enough so that they can stick the insulation /plasterboard to it.

I would have thought bonding would be fine here, but I'd probably use sand /cement

Sorry if my original post wasn't clear - last post had it right, and I've decided to go with a render. Thanks for replies and advice - Tom