I'm about to start plastering the inside of a little garden folly. It doesn't have a damp course, but I don't think it is subject to rising damp. The first 4' is built of reinforced concrete which at one time was sealed with a thin bitumastic compound (whatever they used in the seventies) which is still covering about half the area. There's a ring beam of very dense concrete above that. And above that, the construction is concrete blocks with a concrete render coat, very rough. The outside of the building is concrete rendered.
In the long term, I want to put in some rather fancy plasterwork with moulded designs. I think the first thing would be to create a reasonably uniform plaster surface, then add the fancy stuff. The plaster should be white or white-ish---so it has the look of old lime plaster.
Is there a kind of plaster that will do this job? How do I prepare the various surfaces? I hope that a coat of PVA or SBR (or something else?) would be all that's needed to help new plaster stick on. Or is it better to try to get rid of the black stuff?